Bow grips are very personal. Even the same grips can have little things that are very different about them. Like with a grip on the stick, the middle finger placement could be slightly on the hair to as far back as behind the ferrule on the frog. It depends on the person. Go with whatever feels right.
I've seen Edgar Meyer play a few times and he mostly uses one of the two grips Ken covered already. I believe he's an on the stick or "French grip" guy. This is just speculation from what I have seen. But, on occasion I have seen him play with his thumb on the slide of the frog. Here is a video of him doing it
http://www.youtube.com/watch?v=ZibZ-sB5YJA. He may do this for speed or to try to get flatter hair across the string, I don't know. Does anyone have a clue why he does this?