i have seen just about every kind of grip used but the standard "V" grip is probably most prominant...I used to grip it with my thumb actually up on the handle like a golf grip but that is rare and I never approved of it either and last year forced myself away from it...it still worked and I could do basically everything except a squared off pull shot with that and it made a palm roll much easier since the thumb was already out of the way
Todd Loffredo says, and I figure his opinion counts, that he hold the handle very high up, index finger over the top of the handle touching the rod for control...so if you watch video of him, when he is passing, that is how he hold the rod
when shooting he moves his hand back down the handle (v-form basically) and he says he does this for power...the closer to the end of the handle, the more power...part of the heel of his hand actually sits on the end of the grip
i have messed around with these and have not gotten comfortable with the finger over the top...I am sure it is part mental, getting used to something new, and part that I think it hinders your wrist from full rotation or fluid easy rotation much like my thumb up on the handle did
however, given Todd's sucess and his diligence as a student of the game, I would for sure defer to him and think I probably just am not comfortable enough to know what he knows
now, on the power grip, I do use this and find it is true...at least mentally...I don't know if I really get more power but it feels like it and that is enough