I think it's up to each individual person, whatever makes them most comfortable. Certainly, from time to time I have seen very large breasted women who were braless, and have felt passingly concerned for their backs and shoulders, as well as the harassing male attention most of them would undoubtedly get, but there is no public dress code in this country, thank God! We get to wear what we want to, and should not be judged or condemned by men or other women for doing so...Having said that, I think there are social situations where it is "innapropriate" to dress immodestly and be braless. I'm just a 34B, so about 50% of the time, I comfortably go braless, but once when I was visiting my grandmother at her retirement home, I noticed I was getting shocked stares from right and left, and that everyone seemed very uncomfortable around me. (My grandmother, who doesn't embarrass easily, just told me, "Can't you cover up your Hershy's Kisses, somehow?"--which embarrassed ME) but the majority of people around there were ill-at-ease with me, from my bralessness. There are social incidences such as those where I think wearing a bra, for anyone, large or small, should be a general rule of thumb. It's about respecting other people and situations that you are not the center of.
Every now and then, every few days, I'll get some objectifying and quite undesired attention from men, from being braless,or a rude, pointed look from a woman, and the women's glares always always hurt me most.I know the men are sort of digitally programmed to react like Neo-Neandrathals, to certain signals, but what is with other women putting complete strangers down, judging and condemning them on the manner of their clothing?A bra,or lack of bra, is just that: It does not even begin to tell you the true, whole story of the person.