I would have to agree with Demo, it all depends on the dog. If the dog is growly, but driving in and countering then I am of the opinion the dog is growling more because of the fight. However, if the dog is growling in the grip, pulling away, backs out of the grip (half grip hanging, on by canines) then I would say this is a nerve issue.
It's not so much the growling in my opinion but the other signs the dog is giving off while growling. Does his growling get louder when there's more stress added? Does he back out, or counter in?
These are questions that you have to evaluate for yourself and answer with an open mind. Most people can't fairly evaluate there own dog, especially when it's a question of nerve. But when nerve issue's are recognized only then can you take the proper steps to train accordingly.
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion."