Well, I'm coming into this conversation on the late end, but...
During my church-going days, it really used to bother me. I played in the band for our church so if my nipples were sticking out, everybody would see. Because of that, I used to wear padded bras, even though I didn't need them for size reasons.
Things have changed a lot over the past several years. I'm now not self-conscious about it at all. Last summer my Master made me walk through the grocery store with him wearing jeans, a thin white undershirt, and no bra. I swear, the colour of my nipples was visible, in addition to the shape. But he helped me think of my breasts and nipples as beautiful and sexy, rather than something that should be hidden away.
I don't know why people obsess about it so much. But I do think that in a way, it's repressive to women.