Added a couple of comments 20 November 2014
The general criteria in selecting a beacon frequency is: ........  East coast beacon frequencies should not conflict with European/African beacons......West coast beacons should not conflict with Japanese or "down under".......... and Central USA beacon frequencies should not conflict with Central/South American beacons....... and selecting a frequency within 1 KHz of 28.200MHz should be avoided completely. ........ Beacons of about equal power OK to be close to each other but low power signals and signals of more than 10 Watts should not be on adjacent frequencies.......... maintain at least a 1KHz spacing between beacons in the same North American "quadrant" like Northeast, Southeast, Southwest and Northwest or the same time zone. A 500 Hertz separation OK for beacons located in different time zones or North American "Quadrants".  ........  Past three years has brought many beacons operational........ and with the Brazilian & USA auto controlled beacons restricted to 28.200-28.300 MHz segment of the band it has required the reduction of spacing between signals within the same North American quadrant..... . Beacons considered "grandfathered" are those activated before the appointment of the first IARU Region 2 HF Beacon Coordinator (1 June 2001)...This can/does  cause some overlay of beacon frequencies but, thankfully modern transceivers can easily discern signals within a few Hertz of each other....... update 20 November 2014

THOUGHTS I have  logged many a beacon signal via scatter other than towards the South........ scatter against frontal systems especially...... which usually works best for me towards the back of the "front"........ Frontal system move through our area and proceed towards Southeast USA.......... Often a weather system moves through the area......... and a recent tuning through the propagation beacon band I was able to hear 4 via backscatter... 1 Texas, 1 Georgia and 2 Florida beacons........

I have found that usually if the "transequatorial path" is open there is the possibility for backscatter also  ..... although there are exceptions......

To receive backscatter signals a directional antenna is almost a must......... plus a  person can expect rapid fading and signal flutter..... sometimes it takes listening through the beacon string several times in order to make a positive ID....... I generally hear backscatter signals in the mornings with the antenna Southeast...... South near "high sun" and with the antenna Southwest later in the day........ and sometimes I have heard backscatter signals during evening "greyline".

OK, out of curiosity, why would a person want to operate a propagation beacon?

To provide a service for their fellow Hams… It’s an opportunity to return a bit of their enjoyment to others in the hobby. Ten meters is not an “always on” band and as such a person needs to know when the band is “open”. If a person can hear a QRP propagation beacon it’s a pretty sure thing that the band is open in the direction of the beacon signal.

Beacon Operators are dedicated contributors to propagation studies….. It’s impossible to determine propagation patterns from listening to QSOs… Only a continuous 24/7 beacon operation will provide information that can be used for propagation studies. Always the same power, always the same location & always the same antenna gives a person an idea of the magnitude of any band opening.

A beacon Operator’s Call sign is “on the air” 24 hours a day… It becomes familiar to many Ops because it is heard often…… It’s always nice during a contest or other QSOS for the other person to say, “I heard Your beacon” or “Your Call is certainly familiar”.

There is an unique camaraderie that exists among beacon operators…. They are always willing to help fellow beacon operators with difficulties or resolving problems. When “faulty” operation is noted a quick message with possible causes are Emailed to the beacon Operator. In this respect, beacon operators are kinda “self policing”….. Certainly better than a note from the FCC.

Beacons like repeaters are not cheap or maintenance free….. A very few of the current beacons are “Club owned” and operated….. By far the majority of the ten meter beacons are individual Op operated…. Means that they “foot” the maintenance bills….. they provide a space, antenna and beacon rig.

I’m sure there are additional reasons but these are off the “top of my head”.

Of course, many things enter into determining what frequency
that a beacon is transmitting on. .... first off, there always is a disparity between
manufacturer's digital readouts...... even rigs made by the same
manufacturer will differ..... and many Ops see no need to calibrate their
readout with WWV..... and the "ears" of the listeners are different because
Ops hear different tones better than others... one Op will tune His rig to
hear a 700 Hz tone of a CW signal.... another will want to hear an 800 Hz
tone.. so, in either case..... the digital readout will be different even if
the Ops have correctly calibrated readouts on their rigs.... Plus, other Ops
have rigs with analog readouts..... so the "eye' also becomes a factor in
pin pointing a beacon frequency. ......... So if a beacon is reported on a
frequency different than listed it very possible....as long as it's close.
update 20 November 2014

Of all the beacons that I have in the log that are listed on the same
frequency........ I have never had a problem ID ing both/either when the
conditions are "right" for both/either to be heard.... I have always
considered beacon frequency listings as "ball park" frequencies..... a
person tuning their rig to the listed frequency will be close enough to
"find" the beacon if the band is cooperative.
 

Beacon Operation is often a thankless endeavor ..especially during the low of the sunspot cycle.... and even the E address in the message will get only minimal QSLs...... Most of the
reception reports will come via the HF Beacon Reflector  or the   Beacon Spotter page    
 

Beacon messages.....As this "sunspot" cycle continues on the "downward slide it is important that the message be concise without extraneous information. With the noise, rapid fading and backscatter flutter it is difficult to "wade" through a bunch of information that is not necessary trying to catch the ID of the beacon signal ......... In this "internet age" all of the beacon information is available if the Call is known.

 . Let the beacon do what it was intended to do.......  other than for
historical propagation studies... the main objective of a ten meter propagation beacon is
as a "real time" indicator of whether the band will support a QSO in a given direction.
The average Amateur Radio Operator uses a 100 Watt rig and the SSB mode.

I have a couple of references on my computer that mathematically proves that a 5 Watt CW
signal is the equivalent of a 100 Watt SSB signal. (Given an optimal tone & WPM message)

It's long been my philosophy that for the propagation beacon to be effective in
determining that conditions will support a SSB QSO.... the beacon power should be 10 Watts
or less..... and I'm speaking for North American beacons....... From my listing of 5 June
07 I count 190 North American beacons and 105 for the remainder of the World.

Okay....Where am I coming from....... I've operated a 10 meter beacon for over 15 years.
I've listened/ reported beacons heard to the reflector 3068 times since June 2001

Up until June 1996 (when a medical condition interrupted), I had over 21,000 ten meter
contacts in the log....... many with a single tube 6V6 transmitter (close to 6
Watts)...yes, I still have the logs.
 

The immediate above is AN EXCERPT FROM MY MESSAGE IN RESPONSE TO QRP vs. QRO POWER

I'd like to see.......for North American beacons

At least two beacons in every State and Providence

All beacon antennas omni directional

Max utilization of synchronized beacons on 28.250 & 28.300 MHz (6 Beacons with ID message ten seconds apart)

More 24/7 beacons.... and for those intermittent beacons to have published operating times..... (daylight/weekends/random etc)

E addresses available for all Operators of beacons  (QRZ.COM/BUCKMASTER/message)

Active listening & reporting by the beacon Operators themselves.

Message WPM speed used to separate beacons near the same frequency/North American quadrants.  (like 5 & 15 WPM)

The nucleus of the North American beacons with 500 KC spacing.

Additional beacons activated in Mexico. BINGO!.....XE3D activated 1 January 08 & XE1KK activated 8 August 08

...XE3N activated 26 April 2010