For a very good explanation of how dominion spreads, check out this link. It is the best documentation I have seen for Dominions ever and it is a shame that the individual did not finish the strategy guide. I believe that a number of the earlier posters would be interested to compare their understanding of dominion to this guide.
http://www.maladjustite.com/dominion.pdf
Two quick things from the guide.
Preaching with priests only effects the province the priest is in, it can not spread outward.
Low dominion would spread faster than high dominion at first, but will spread more slowly when it encounters enemy dominion and tries to overcome it. Further, low dominion is more easily pushed back than high dominion. This is a combination of both your pretenders dominion and the dominion of the province in question.
From my own thoughts, you should consider how big the map is and how many temples you plan to build. In a huge map, you may build 25 castles, which would boost your dominion five points. On a small map, you would not be able to do the same.
If you go for a fast spread dominion, perhaps as part of a rapid expansion plan against indies with an immortal or SC, you will probably want to ensure you are building lots of temples before the mid game. This is to ensure that your dominion spreads faster and boosts it later, so you do not lose the dominion you spread early on.
When picking your starting dominion, you need to bear in mind that you can raise it, and then decide how important it is to you. Also, I believe that dominion maxs out at 10. I could be wrong, but if I am right, you probably do not want to start any higher than 8. After all, most people will build at least ten temples. If anyone can confirm this, please let me know.