Not many gals can go without, in my observation. A few might, but gravity can do unkind things even to the small ones, especially if you have had children. My cousin is an A cup, and she said that what little she had before has all gone deflated now, after having borne two children, and now they are like "sad little dog's ears". And she DIDN'T breastfeed!
I'm not in the "everyone MUST wear a bra" club because I think if you truly can go without, they why not -- but I tell ya, mine have always needed support. I dislike jiggling.
ETA I think breast sagging has a lot to do with genetics. I don't wear a bra to sleep, although I did when I was pregnant and breastfeeding because they got so huge and heavy, and I thought good grief, these are going to be down to my knees by the time I'm finished. Well, they're not. I have hormone-sensitive breast tissue, which means it responds to cyclic progesterone surges by plumping up (somewhat painfully, I might add) -- but the upside is that even though I do not have a tiny bosom, it's still pretty perky, and I am no longer young.