I like Dr. Phil's advice on this one, "You have to avoid even the appearance of impropriety".

So regardless of whether or not flirting is harmless, you have to avoid that appearance in a relationship. It often starts to cause doubt and from doubt come trust issues and then resentment. So in my opinion neither of you should be flirting in any sort of way. It's best to avoid a dumb ego-boost like flirting and focus on the real relationship.

Oh, and from resentment comes the dark side ... I just realized what I wrote above.