I did some testing too with U's, and dropped from 1000, 2000, 2700, 3200, 3700 meters and really didn't see much difference between, well, between any of them. Also trying to drop them from higher altitude just makes aiming so hard due to lack of proper targeting system, that you are better off with bombing from 1000 meters, which I guess answers my original question then; "NO! Don't use any of them"