If you are a transvestite and someone you feel is heterosexual is hitting on you, at what point do you feel you should inform them that "you can't always judge a book by its cover."?

Should you do it, as soon as you feel they are hitting on you? How about when they ask you out? Or maybe right before they pull your pants down?

At what point (if any) do you feel you should be obligated to tell your true gender?

And why do you feel that this is the best time?