Personally... I have Ds, and they don't give me any back pain - I have it all in FRONT. I cannot NOT wear a bra, because it feels like my breast is basically hanging from the skin, not decently attached with the 'breast wall' or whatever the hell that's called, and it feels full of pulling and heavy and tearing. Horrible. It doesn't bother me if I'm laying down or whatever, and I can leave off a bra for a little while, but seriously, I HATE not having one on for this reason... Also, my breasts and nipples are really not sensitive at all. Well, at least where pleasure is concerned.
Howeeeeeever, I have a genetic condition, that affects my skin, tendons/ligaments, cartilage, fascha (or however you spell that), so my set up is radically different than other people's. Also it comes with a pain multiplier. YAY!
On the other hand, my girlfriend has Gs, and she used to have HORRENDOUS back pain... in her case, it turned out she had a gall bladder about to an hero. After that was removed, sooo much less pain. I'm not really sure what it's like for her in front... She's been living on maybe three bras for the past couple years; it's awful. Too damn expensive. I'm not actually certain how sensitive her breasts are overall, but her nipples are CRAZY sensitive - I play with them a lot with her.
Unfortunately, I also accidently put my elbow on them quite frequently when we're sharing a bed.
... Uh, holy shit, wall of text.
> tl;dr - ... ... bump?