I've had three shepherds and an Aussie too, and only the Aussie and one of the shepherds have been truly mouthy. The Aussie had to be taught not to nip - she came to me at a year old as a rescue and had pretty much run the household before then. Biting was not something she had a lot of inhibition about. But I taught her to only grab something on command. She LOVED grabbing the pant leg and tugging, and I could actually drag her around the room that way .. *LOL* .. but by putting it on cue, it gave her much more control and she would wait (eagerly) for the cue/command. I could send her to someone else to do it, too. She was the dog we used in herding if we needed to move a recalcitrant sheep. Lady (SUCH a misnomer!) had no problems going in and using a full bite on a sheep that wouldn't move.
My current shepherd, Trick, has to have something in her mouth if she gets excited. Her half-brother (owned by my Mom) is the same exact way, and I think it must be genetic. Both get excited and then they want to use their mouths and have learned to grab up a toy before they greet someone. It keeps them safe!
I was at a herding seminar once and was fascinated by a border collie who had to have something in his mouth as he herded. His focus was so strong on the sheep, but he would grab up a stick or a bunch of grass or even a mouthful of dirt .. the handler said he did that in order to prevent himself from biting at the sheep! It's amazing how our dogs learn coping mechanisms to stop themselves from behaviors we humans deem "wrong".
Melanie and the gang in Alaska