Vis-a-vis Ed's point re: terrain:

I tend to roll out with my tires over-inflated, and back off PSI as the current conditions require.

I'm a fat guy, so i start out at 45, knowing i'm gonna end up somewhere just under 40. Minus around 10psi on my 29"x2.55s.

If i'm taking the knobbies to the pavement, i inflate to the max labeled on the rubber.

-rob

ps this thread makes an excellent point. reviews/article talking about optimal tire pressure are basically worthless