NON- VIOLENT protection. Lots of service dogs are trained for it. Look it up.
I don't need to look it up. I'm familiar with it. My Father-in-law works with service dogs in training. If you need a dog to prevent you from self-mutiliating, remind you to take medication, check a room for occupants, or protect you from injuring yourself when you have a seizure - I'm all in favor. If your dog acts as a physical barrier between you and danger - that's great.
When your dog starts looking or acting aggressive (and growling counts) - it's not OK. I understand that it is comforting to you that your dog is so protective. The fact remains that the US Justice department has clarified that growling (and you're realistically talking about a threatening growl) is not acceptable behavior from a service dog.
This is an issue for you and your trainer not you and people who don't know your dog. You'll need to identify what situations might be problematic. Watch youtube videos, look at crowd pictures and try to come up with plans.