The earlier the better because once they reach those terrible twos and you havent got a handle on them your are in big trouble.
I never had to smack mine, although i did tap the back of their hands as toddlers to help teach them right from wrong or if they went through a biting stage as many do.
I simply used a firm voice, taught them manners and respect and i listened to them when something was bothering them, so im blessed to say i have always had a good relationship with my kids xxx