A growl is a valuable means of communication for a dog - something to be appreciated and respected. Never punish your dog for growling. You need to know when he's uncomfortable so he's not pushed past his ability to cope. Punishment can suppress the growl, eliminating the warning that he's about to snap or worse. Also, punishing a growling, upset dog can escalate him into full-on aggression.  

If your dog growls:  

Don't forget, play-growling is perfectly acceptable. As long as you're sure he's playing, there's no need to modify this behavior.