I usually ran my old 35" BFG MT around 10-12 psi with 15x8 & 15 x10's. But one run I did bend (3) 15x8 steelies at 11 psi. I have had them as low as 5 psi in the snow with no problems.
My 36 IROK's seem to really start to hook up about 5 psi or so. Just got back from a snow run with 50 miles of snow from 12" - 24" of wet crappy slushy stuff at 3 1/2 psi and didn't have any problems with my 15 x10 steelies.