what kind of terrain?
how fast do you intend to go?
what kind of tires?
I air down to between 20 and 8 psi, depending on the terrain. 20 for long dirt trails hwhere I'll go faster, for some comfort. 15 or so for general offroad use. 12 for wheeling on the rocks, 8 for wheeling in real 'sand'.
I'm on MT/R's with a good sidewall.