i run 31"x10.5" on 15"x8" rims. have had no problems so far, and the MT/Rs are a pretty good all rounder, as there is loose dirt/gravel, mud, sand, hills (loose and rocky) and mud. did i mention mud as well?
anyway, where i am, there's not a whole heap of bog rocks and boulders as you would find in rock crawling, so obviously i dont do it, i'd love to though.
i should've got 32"s, as i have enough room for them to fit and not rub on everything. but for what i'm doing, my vit is sweet.