Gosh, does anyone use titles any more? It's a preference or a custom, not anything legal. You can't use a title like Dr that indicates a qualification unless you actually have the corresponding qualification. But anyone can be a Mr, Miss, Mrs, or Ms.
It seems logical to me that the title should match the name you are using. It would be a bit confusing to be known as "Mr. Diane Smith", for example. When you change your name, that would be a good time to indicate your preference for the title.