Wood is an organic substance, every piece is different. I have had quite a bit of success bending stocks, especially bending for cast. Do they sometimes move after a bend? Indeed, they do. Is that the most common outcome? Not in my experience. I would far rather set up a good candidate for bending in the jig and see if I can get the desired results than take a rasp to the comb of a gun.
I have been bending guns for over 20 years, if it was not successful the vast majority of the time, I would have stopped offering the service years ago.