[Nick_K]

I don't think you can rely on dominion kills... even the AI can usually prevent them. Sometimes an AI will have a low dominion though and will just die out.
You need to be careful it doesn't happen to you, of course. Build temples and preach if you see your dominion getting too low. If you start next to a couple of enemies with high dominion things can get a little troublesome early on.

My main use of dominion kills is to avoid storming heavily defended castles. If I conquer most of an enemies' provinces and build temples there then I can preach in a few besieged provinces until his dominion dies.