Some comes down to preference visually. I remember a guy that used to post here a lot than ran some 35" Boggers on 15" x 14" wide rims. I thought it looked stupid, but it wasn't my truck. It did make the tires look even wider than they already were.
Here's my really loose guidelines...aywhere up to 4" narrower than section width is usually fine for the diameter of tires we run (31-44") There is more than enough sidewall available to compensate.
For the larger and wider end of the tire scale you can go even narrower. It's pretty common to see a 15" or wider tire in the 40-44" range on the Hummer H1 rims which are ~9" wide...
Example, a 31 x 10.5 is fine on a 6" rim, ideal on a 7" rim IMO, and still fine on an 8" rim.
Erring on the narrower side of things helps keep the tires on the rims at lower air pressures.
I don't know if that helps what you're trying to do, but that's been my experience. Tires with much less sidewall you do have to stick much closer to recomended rim widths, and the less sidewall, the less wiggle room in that regard.
Rene