I'm running 33x12.5/15s on 8" steelies. To me, the tire width vs. rim width seems about perfect - the rubber sticks out about only 2" per side of the wheel, and as folks said, when you lower air pressure it helps keep the tires on the wheels. Mine stick out past the fender about 2" or so, but as it's been said, that's not a bad thing (and you can always put on flares).

On low-profile tires (40/50/60 series), you definitely want to be much closer in rim width and tire width - but with offroad tires like these, with huge sidewalls and the way they're designed to fit/flex, I think you want a decent difference in rim width & tire width. (33" tires on 15" rims have more sidewall to them than the rim width!). 10" rims with 12.5" tires might have the sidewalls too vertical, yeilding not enough bulge/flex, and would make the ride harsher, IMHO.

HTH,
Tom