[BigDisAwesome]

I start the game with only 1 pd per province to save money. Then I like to give it all a bump to 10 to catch stealthers and for Call of the Winds attacks.
If I have a hostile border I'll try to get about 20 pd and hire some deer tribe archers and maybe drop a mage there for good measure. I generally don't do that very often though, as I like to have NAP's with people.

The only time I ever go above 10 is if I have some extra money and no army where I'm expecting to get attacked next turn. This generally only works if the army attacking you is a small raiding force though. But if your pd gets slingers or archers they can be devastating against a small force of troops not expecting much.