I've always used my index finger, but see so many writers say that you can only get power when using the thumb on top. What a fishing author says doesn't bother me too much, but I tried using my thumb a few days ago, and it was actually painful! I have no problems casting with my index finger on top, and can fish all day like that. Charles Ritz insisted that the thumb must be used on top of the grib to get distance, but also said that using the index finger provided greater accuracy on short casts. I was looking at a video of Lee Wulff fishing for salmon, and he uses his index finger on top:
Lee's wife Joan is also a big advocate of the thumb being on top, so I wonder if they ever debated the issue. Lee looks to have been more of the type of guy to just go fishing, however.