[trebuchet]

I've found that the new bless effects make a difference when I choose, keeping in mind that your prophet seems automatically/permanently blessed. That is, assuming that the units in contention aren't all sacred anyway (which is sometimes the case). I am not big on combat prophets, but with certain bless effects this becomes a much more interesting possibility.

Unless there's a compelling reason to wait (like an uberpriest available for boosting a turn later) I will often put it on one of my starting commanders on the first turn - no matter how bad they are - to increase my dominion faster in the early game.

With Ermor I try to wait until I have the most powerful base unholy priest available, just to be able to reanimate more powerful undead.