What I'm going to tell you is probably absolutly wrong but usually don't try and run wheels wider than my treadwidth. That said an L78 should run fine on a 10" rim.
The rears in this pic are L78's on 8" rims... they may fit on a wider rim, but they won't look right. I agree that best rule of thumb is to use the tread width to determine rim width if you want them to look decent.
According to the Tire and Rim Association books, the approved rims for a L78-15 is 6, 6 1/2, 7, 8. Now...sometimes its fun just to break the rules! The dilemma you get into with running wider rims than recommended is funky tire wear not to mention the funky way the tire looks.