Disclaimer: This is only based on a few sources I've read online. A person who's actually experienced MtF hormone replacement therapy will be much more qualified to answer your question.
From what I've read, it looks like any weight gain you experience while on HRT will go to where-ever it should be for a woman (talking MtF) or a man (FtM), but most of the fat you have already will stay where it is.
Granted, your body's going to be changing a lot with this, and maybe my sources are a little off or downright incorrect.
With that information, though, I'd try to lose weight now, especially if you want to slim down to a more androgynous or feminine figure, so if you start gaining weight after starting HRT it'll be in the right places.
*Shrugs* My take on it; you could always ask Google and see what you get from there.