If bras are in any way uncomfortable for you, or the straps slide down at all when not tightened fully, or you can fit a fist (or worse, TWO fists!) under the bra band, or you have armpit rolls, or you have the dreaded quadriboob (to test for that, push ALL your breast tissue into the bras, including any armpit rolls and excess back fat-that's probably all breast tissue!), or if your breasts get squished by bras, you're wearing the wrong size. There's a MAJOR misconception out there that you have to add a number to the underbust to get the band size (you don't-your underbust measurement is your band size), or if you have sagging breasts, or if bras and that the cup size is unrelated to the band size (a 28F has smaller cups than a 38C). I figure this would be especially important for transwomen for similar reasons why it's especially important for younger girls-their breast tissue is still growing.
I recommend you check out Eternal*Voyageur, Thin and Curvy, Bras I Hate, Braless in Brazil, and the other bra blogs along the same lines. They have heaps of really helpful information.