There's a trail riding collumn in Australasian Dirt Bike mag written by Stephen Gall (the ex-champion motocrosser). Recently he recommended that the minimum pressure that will avoid damage from rocks is 14 psi.
So, I run 14 psi generally. Under really muddy, soft conditions such as we've been having through winter, I drop the tyres down to about 12 for the sake of traction, but try to keep an eye out for sharp rocks.
By the way, you can buy special low-pressure pencil-type tyre gauges (0-20 psi) in Super Cheap Auto stores that are more accurate in this sort of range. I carry one with me while riding along with a plastic bicycle pump so that I can adjust my tyre pressures if necessary.
Cheers