If you are very concerned about aesthetics, you might want to go on T first and see how your pecs develop. Then the surgeon might do a better job of sculpting your chest. But I dunno--guys go both ways and even in between. I was on T for only a few months--under five months--when I had top surgery.
I can't imagine that you would have any problems with regrowth unless the surgeon leaves too much tissue behind and you go on some kind of uber-estrogen kick, which I suspect you are not about to do.
Some surgeons do ask you to suspend T use for a period before the surgery. If you are not on T yet, you won't have to worry about that. I suppose that's one advantage to starting T afterward.