Well, I went ahead and bought a couple of wal-mart nursing bras early. I bought them one size up, but I am wearing it now. I decided that today I was not just not in the mood for underwire, and I today I really don't care if my bra doesn't fit perfect. (Geesh, in the mood I am in, they are lucky I am wearing one at all! )
With DS, I did experience some pain from underwire as I got close to delivery and definitely afterward. I think I might be one of the ladies prone to clogged ducts!
Based on my experience with DS, I believe I will be able to keep wearing these bras while nursing DS. (***keeping fingers crossed that I am right***)
I experienced engorgement a few times with DS...as Lida says, a couple days after birth and then anytime I went too long between feedings. (We traveled a lot to show off my baby after the first month...and usually that is when I would try to push time between feedings...suddenly super-big, hard boobs!) Engorgement is something you will want to deal with (via feeding or pumping) because it can get really uncomfortable if you don't, s o there is no need to buy a bra just to fit engorged breasts.
For the first weeks, I lived in nursing camisoles, except at night when I would just wear a tank top. I never purchased sleeping bras, but that is mainly because I have never been comfortable wearing anything tight to sleep.
One thing I found invaluable - nursing pads. I would leak at the drop of a hat -- my cat would meow for food, and out comes the milk!