First off, none of the three Schnauzers raised in my house have ever growled at me once, except in play. And the only one to do that is Pukalu. And probably because I encourage it, though I'm not trying to.
I taught Muka not to bark so well that I don't think I'll ever be able to get him to "speak" on command. But I decided to try to teach that to Pukalu. I have him speaking on command pretty well now, but it's more often a growl and a growling bark than the cute bark I'm looking for. We're working on it.
Also, all three dogs growl when playing with each other. It was really hard for me to get used to that and accept that it wasn't something mean as I'm new to owning a dog. Or even being around them.
I have the same problem with the mouthing getting mixed in with the rough play. I would be really concerned about it, except for the fact that all three dogs have done it but never mouthed anyone else, namely my grandchildren. Probably not because they know they shouldn't, but because I don't let them play roughly with the dogs. They would be very afraid if a dog growled at them, too.
There are certainly a lot of people who would say that a dog should never put its teeth on a person, and I would not say they are wrong. But I have allowed it with three dogs now, and I think it's been fine for us. I think dogs have a great ability to act differently in different circumstances, and that's why I feel safe letting them mouth me... because I have never seen them do it to someone else.
I think they know not to. Or perhaps I'm reading too much into it, and it's simply that they associate mouthing with rough play and I don't allow anyone else to play roughly with them.
I agree that grabbing at your pants leg should be a time out.