I have no serious problem being sir'd, because on the side of other people, it is all about perception. If you are an MtF trans, it is all about history and progress. However, it can hurt to be sir'd if you expect your transition to have a positive effect to the point you think you should pass everytime.
It is all about perception. Remember that the shop assistant os bound to what he/she sees. You are more than that, not just a man, not just a woman. You can work on it, work on your appearance, work on your body, work on everything you want, to have better osmosis with yourself.