The importance for a contester, how frequent to ID, is of paramount importance to his final score.
When people say, a contest DX station should sign every QSO, they don’t know what they are talking about, and should know better.

Let’s imagine the following situation:

ZD8Z from Ascension during CQWW CW contest does 8000 QSO’s, at a 166,77 hour QSO rate, signing his callsign every QSO.
He works 170 zones, 550 countries with a 2,97 average QSO point for a final score of 17,107,200 points.
In the same contest there are four ZD8Z clones:

ZD8Z clone#1 signs his call every two QSO’s (1,0,1,0,….)
ZD8Z clone#2 signs his call every three QSO’s (1,0,0,1,0,0,1,…)
ZD8Z clone#3 signs his call every four QSO’s (1,0,0,0,1,0,0,0,1,…)
ZD8Z clone#4 signs his call every five QSO’s (1,0,0,0,0,1,0,0,0,0,1,…)

With the help of an audio spectrum analyzer, it can be determined that ZD8Z @ 40 WPM lasts exactly 1,655 seconds.

Let’s determine how much time each operator spends in a 48-hour contest transmitting his call:

ZD8Z 1,655*8000=13240 seconds = 3,68 hours
ZD8Z clone#1 1,655*4000=6620 seconds = 1,84 hours, i.e. saves 1,84 hours
ZD8Z clone#2 1,655*2667=4414 seconds = 1,23 hours, i.e. saves 2,45 hours
ZD8Z clone#3 1,655*2000=3310 seconds = 0,92 hours, i.e. saves 2,76 hours
ZD8Z clone#4 1,655*1600=2648 seconds = 0,74 hours, i.e. saves 2,94 hours

ZD8Z is working at a 166,67 hour rate (8000/48), the same rate as the clones.
But the clones, because of the time they saved, not signing every QSO, will work more QSO’s in the 48 hour period:

ZD8Z 8000 QSO
ZD8Z clone#1 8000+306 QSO’s
ZD8Z clone#2 8000+409 QSO’s
ZD8Z clone#3 8000+460 QSO’s
ZD8Z clone#4 8000+490 QSO’s

So the final score will be:

ZD8Z clone#4 18.155.808 (8490/2,97/170/550)
ZD8Z clone#3 18.090.270
ZD8Z clone#2 17.980.985
ZD8Z clone#1 17.762.580
ZD8Z 17.170.200

ZD8Z can be admired for signing every QSO, but his clones will beat him anytime.

The decision making process how frequent to ID has some variables:

How long since last ID
The instant rate
The number of stations calling in the pile-up
The number of stations coming to the pile-up

The combination of these four variables will give the contest operator all the information he needs, when to ID next.

Signing every QSO can be the worse decision and the best decision, depending on the situation. From a DX location, with constant pile-ups for 48 hours, signing every QSO is poor operating practice: not only it will reduce the contest DX station score but it will prevent the DX contest station to work more contest stations. Of course being for very long periods of time without ID is also poor operating practice.

It is my experience that IDing at least every minute or until someone sends “?” is a good way to balance the needs of the contest DX station and the needs of those arriving to the pile-up and unaware of the contest station callsign.