I teach and train thumbs forward as a foundation for all grips to lock the hands in, and then the weak hand thumb either finds its rest along the frame or curls back to thumb the hammer or be out of the way.
With my shooting being 2/3 semi-automatic and 1/3 revolver I need to think when I grip. Palms on palms, fingers on fingers, locks the grip in the hands. This puts thumbs forward, so if that puts one thumb in danger from the cylinder it is just a slight twist to bring that thumb over the top.