At Mas Ayoob's LFI-II class in 1989, I was shooting a Smith M66 3" with my left thumb wrapped around the back of the gun, as I always had. One of his assistant instructors came by and said to me, "If you ever shoot a semiauto like that, you'll rip your left thumb off."
"Yes, I know," I said. "That's why I don't shoot a semiauto like that."
Oddly, for many years I shot revolvers with my left index finger riding on the front of the trigger guard, the way many people used to shoot semiautos. I finally for some reason realized that this wasn't the strongest technique, and switched that finger to under the trigger guard--I believe this is called the "Ayoob wedge" grip IIRC. I also usually now shoot revolvers with my right thumb down and my left thumb over top of it, but sometimes still revert to the left-thumb-around-back old way. Doesn't seem to make much difference for me.
I shoot a semiauto both thumbs pointing forward, taking care NOT to touch either the slide nor the slide stop.