Flam, you do not need to be on T for a good surgery result. As I mentioned above, I did mine pre-T and I still ended up with a completely masculine chest. I could not be happier with the results. From surgery alone, I went from being read as female 100% of the time by strangers to being read as male roughly 50% of the time; I blame the other 50% on my 'pretty' girlish facial features.
Keep in mind that it's harder for a body on estrogen to build muscles, especially upper body muscles. Also, estrogen pads the body with more fat, which results in a softer look. But if you like that, then there's no reason that you MUST go on T. I would recommend telling your surgeon that you don't plan on HRT, though, it may be something they need to know when doing the reconstruction (I honestly don't know, but it's better to find out, right?).