what willman said. the narrow rim will help grip the bead--i run a 12.5" wide on 8" rim for that reason--but under 12 you risk losing it.
different tires flex their sidewall at different points...dont air down as much if you see that your tire is flexing near the rim. a quality tire intended to serve offroad will flex the sidewall closer to the tread, using the side lugs properly and minimizing the heat buildup and wear in the sidewall. when you're airing down, see if the tire balloons as it loses pressure, or puddles near the tread. i cant think of a better way to describe it, it really does look that way--either it looks like a balloon would if you pressed it against a hard surface, squishing out in the middle, or it looks more like a very viscous fluid being poured on a flat surface.
your air-down pressure really depends on how much rubber you have between rim and road, and how much your vehicle weighs, but 15# is a good all-around number for most situations. i've run trails aired down to 25#, but had a lot of weight in the bed at the time...cold the tire was at 40#.
-sean
