I agree with Tom of Sweden, in that it is good (vital, I would add) to see things from your partner's point of view. But it would seem to me that if you had a sub for a mentor you probably wouldn't play with that person. They would be there to answer questions and to debrief after a session, etc.

If you do have someone acting as mentor and sub it would probably be best to have them make comments before and after a session and to follow you during. That would prevent topping from the bottom.