Well, you can't wear anything white or with really thin fabric... as far as nipping out, it's gonna happen, it's just whether you can get used to it and not care. And you want to wear a nice enough outfit that you look like you planned it, not like you're just lazy and didn't get dressed. Plenty of celebrities do it, like nobody could have been wearing a bra with most of the dresses at the Oscars.
My boobs aren't super big but I don't go bra less unless I'm wearing a sweatshirt haha I just think bras give my boobs a nicer shape. If you were to layer a clingy tank top under a flowy one (when it's warm out obviously) i think that would probably look fine. You could probably also find good suggestions on Google hahah
I'm a 36 C or D depending on the bra, and I'm also very firm. When I go braless I prefer a button-front blouse/miniskirt combo, I get my blouses fitted nicely, not too tight or too loose, sometimes with pockets, sometimes without, and my nipples can get a bit "friendly". I just go with it and smile, I'm proud of what I've got and confident in showing it a bit.