thought i'd dispell a myth - whether or not you wear a bra has little or no effect on sagging later in life. sagging has to do with hormones (or lack there of from old age) and whether or not you breastfeed and for how long. there are hormones in younger (mainly adolescent and early 20's) women that are no longer in older women, and the constant draining and filling of the milk ducts when you nurse massages the denser breast tissue into a softer, more palpable consistency, much like getting a massage to rid your thighs of cellulite. i'm a heavy advocate for breastfeeding (i nursed both my kids, the first until nearly age two and the second i only stopped earlier due to some meds i had to be on...) however i can definately say i sometimes cry about the look/size of my breasts now (from a nice firm, perky 34DD to a now saggy pathetic 34-barely-C-usually-B).
as far as whether or not you choose to wear a bra - i rarely wear one anymore. i only wear one when i need/want them to look bigger/firmer/perkier than they are (sad to say). i guess it depends on your own comfort level. however i used to always wear one because they used to be so big i would get backaches and even headaches if i didn't wear a bra... now the bra itself causes the discomfort and to the breasts no less, not the other way around... now, though, i find a nice alterntive if i need to be 'conservative' is those tank tops with the "built in shelf-bra". comfy and yet some support and nipple coverage.