I agree SH, that a good whack cures a smart-mouth, and a spanking is something that kids don't forget, but there's also little doubt in my mind that you don't have to go too far to have gone "too far".
My dad used a belt, on very infrequent occasions when we were very young, but if he had tried it when I was bigger, I'd have killed him. Literally.
It's been a point of contention for years, actually, and he once tried to deny it and justify it in the same conversation.
Haven't talked to him since.