Right hand index finger for the left button, middle finger on the right button, and I do a lot of right clicks. I've tried using the wheel with the middle finger but it doesn't work well for me. I scroll often, but never use the wheel as a button.
I also tried using the wheel with the middle finger, after seeing someone else using their mouse like that. But it felt very wrong to me, so I got curious how common this "other" way was.