I do not want saggy boobs! I always wear a bra at night, esp while pregnant even though normally I'm a B cup. Luckily it works because my boobs are as perky as before my last pregnancy and I bf'ed for a year.
I think that has more to do with luck and genetics. I've worn a bra 24/7 since I was 12 and let's just say the results are not the same! Wouldn't trade it, but most people can't prevent changes in their breasts from pregnancy.