For a long time, I've been "clothing" my prim breasts the same way most "clothing" in SL works. Aside from attachments, people in SL walk around like those models with body paint designed to look like clothing.
Unfortunately for SL, clothing doesn't work that way. Real clothing is a separate object from one's body, and it assumes the position of least energy. In particular, it jumps over cleavage. Shirts and blouses look very, very wrong in SL—text on T-shirts is illegible and images on them distorted past recognition because they're just textures painted on the body.
Ironically, the only tops that look right in SL are those that are designed to work with prim breasts. That clothing layer can be made to take the shape that actual clothing would take. JIRA entry VWR-1080 asks that something be done about cleavage and clothing for the standard avatar, but it's languished since its proposal nearly three years ago, with no comments since June 2009.
For a long time, I've been rationalizing, because I was doing just what non-prim breast users in SL do. Now that I have prim breasts with a clothing layer, the increased verisimilitude is so much better:
I cannot thank the creators of prim breasts enough.