what the others said about winning and losing tug.
As long as your dog brings you the tug back and/or releases it when you ask - you're in charge in every way that matters.
Training that involves lots of positive re-inforcement ie getting it right a lot and getting praise/pats/treats/games of tug (lots of rewards) is great for building a dog's confidence. The more they can do what you want and know they're getting it right - the better. Ie when you're training, make it easy for them to get right when they're learning...