Personally, I like between 30 and 45 degrees. If you use too little, it looks like you tried to do it level, and didn't quite get it right. I've seen pictures on this forum of just about everything though. Some of us like to consider the cant of a holster when deciding. Something like a belt or checkbook cover, etc., it wouldn't matter.
(I guess the thing is to use the same angle for the entire piece, though!)