Well, I can tell you what works for me. I'm a pretty good sidearmer--it's been my main shot since I started, though I'm trying to wean myself of it. The grip I use, I basically make a peace sign with my fingers, the middle finger goes inside the rim of the disc, and the index finger more or less points towards the middle. My thumb pretty much rests between my fingers on top of the disc (if the disc weren't there, it'd be in perfect postion to pull off the good ol' "stole your nose trick"). I've seen some people squeeze the disc with their thumb, but I pretty much just have it there to keep the disc from flipping out of my hand.
I think with my grip, I'm not getting as much potential power out of my drives as I could, but it's what I'm used to, and found it to be by far the most accurate of the grips I've experimented with.
This is a pretty good little video to watch: http://www.youtube.com/watch?v=DOECjLjhiTI