I get this question quite often, and I always ask people to clarify why they are concerned that this might be a problem. They usually tell me that they heard from someone else or saw a television show that talks about "putting dogs in their place" or making sure dogs do not become the "alpha" in the house.
Dog Training Blog
Did you know that there are different styles of training within the positive reinforcement "camp"? There are trainers that only use one style and others (like myself) use many different strategies depending on what works in a given situation. However, I NEVER use pain or fear when training.
There are a lot of misconceptions about playing tug with your dog. In my experience, (and lots of other trainers I respect) as long as you teach your dog some ground rules, tug is a great way to condition your dog to have a soft mouth as well as providing your dog exercise.