My doctor told me that the breast size actually will change often, such as when you're on your menstrual cycle. Also, very seldom do both breasts have the exact same size (same for your feet). Thus, it's really important to have a frequent professional bra fitting to purchase the right sizes - in reality, we should have different sizes for different times of the month.
With that said, I HATE bras. HATE, HATE, HATE. I seriously will do a bra-burning riot one of these days. Yet, if I don't wear it, I'm likely to get fired or something.
I also firmly, with all of my heart and soul, believe that bras contribute to breast cancer and other issues such as shoulder pain. There's no proof, no data to back me up...but later on in the years, I bet that will be a factor they discover.