in fact, doctors recommend that you go off T at least 4 weeks before surgery because it may lead to excessive bleeding during surgery.
i had top surgery and am not/never was on T. (In my very personal opinion, it does make sense to get the surgery before the T-induced fat redistribution takes place, because they'll do liposuction as well and that way you can decide to go off T later without having to fear regrowth.)
While some people's boobs shrink on T, that doesn't usually affect the choice of surgery method (say you shrink from a C cup to an A cup, then there'll be a lot of loose skin, so you'd probably still need to go with DI).
Also, the breast tissue becomes more dense on T, but again, that doesn't really affect surgery.