Originally Posted by
AlmostTrick
It would probably be better to ask the particular headlight manufactures, but don't most bike lights have a beam that is designed to operate in a particular position, (usually upright) and wouldn't flipping it upside down compromise this? I suspect cheap lights are not designed with a top and bottom but would be surprised if high end lights weren't.
I think very few lights in the North American market, at any price, have shaped lenses which would make mounting orientation problematic. The German STVZO standard does require a cut-off in the lens and are shaped/directional in that way, and would not work well upside down. There are few of those here in USA, though.