You want the widest rim you can get that will fit. It flattens out the tread giving you a better footprint. This is especially true for radial tires. However if you go to wide it looks weird.
For a 10.50 section width tire i'd run 8 or 8.5" rim.
My 12.50's (13.5 section width) are on 11.25's adn that is about the max i'd go