I have so many thoughts about this, and I don't know what my final conclusion is. After nursing for 13 years, I don't think pregnancy and bf'ing have done much, if anything, to necessitate a bra as far as size or sagging. Rapid weight gain (stretch marks) certainly did its damage and slow but substantial weight gain did the most. Breast size changes seem to have the most to do with body fat percentage for me, and most, I think.
The biggest reason I don't care to go braless is being nipply, and I think that *may* have to do with breastfeeding. It's hard to conceal, even through a bra & sweater, unless the bra is pretty thick.
As far as comfort, I hate doing anything other than walking w/o a bra. Can't bound up the stairs or jump up quickly or anything. But I don't know how much that would change if you never wore one. I saw a video from the series Meet the Natives where the people of Vanuatu were all bouncing in unison for a long time. It was hard to tell, but the it looked like the (topless) women were supporting their breasts with one hand. No, their breasts didn't hit their knees. They didn't stand out like porn stars or rocket ladies from the 50s, but they probably didn't even come anywhere close to the navel. http://www.galen-frysinger.com/graphics/vanuatu2.jpg
That said, I feel bitter at times that (so far as I know), female human beings are the only animal which has breasts all the time. Doesn't seem quite fair.
I think this works for small breasts, but for larger, the monoboob is inevitable with a shelf bra, and that is just SO unflattering in my eyes. Can't go there.I do wear dancer's tanks that have a "bra" in them, which is what the site above recommends if feel you need a little more support.