My advanced instructor was also a pattern pilot, he was a thumbs guy. When I asked him about it he just said to use whatever worked for me. From day one I was a pincher. My thumbs are so short and fat I couldn't use just thumbs if I wanted to. It just doesn't feel right and I can't do it anyway. I do tend to add in some right aileron when I'm pulling straight back so I have gotten into letting off the thumb pressure and thinking about pulling straight back with my index finger when I do vertical movements. As just a sport pilot it's not a big deal but I do like to start and finish a loop in the same spot.