Tug is one of Honey's favourite games... but we've always played it with rules... when she's given the ok to tuggie, she'll give it her all, but we can ask her to stop, and she will. She also knows that if teeth accidentally touch skin, play stops immediately.
She will also do formal retrieves with her tug toys - it all depends what we ask her to do!
I don't think playing tug destroys a "soft mouth" - though fully understand it's not the game to play with a pup who is going to work...