I do if I am wearing a something that a bra would interfere with like a low back dress/shirt, a strapless dress or something like that. In the summer I tend to skip them more but I usually I wear one. I like lingerie.
I never go without a bra unless I'm staying at home because I'm well endowed with D cups so it isn't very comfortable and would draw even more attention in public.
I may occasionally run to the store without one, I'm pretty big (34D) but I can still pull it off without it looking sloppy, LOL. But I usually where one depending on the outfit I'm wearing. Some things don't need one(built in support) and some just wont allow for it without it looking tacky.
I do it at home though, but not very often because gravity.
And then sometimes I'll lay on my tummy and then pinch the hell out of my nips on whatever surface I'm on. It's a complicated big boob problem...and it hurts like shit.
When I'm at home or on vacation, around friends or wanting to look and act slutty, I don't wear a bra. When I'm at work or at a work-related function or at some gathering where a bra is appropriate, then I wear a bra. However, I love the feeling of not having a bra on and I'm a 36 D.