I agree with Jannet, it's up to the individual.  If you're not comfortable with how things appear in what you wear, then it's probably time for a change.  For me, as long as I wear something beneath my T-Shirts, then I am okay, but I can't wear a T-Shirt by itself.  If I do, my nipples protrude right through my shirt, and draw much unneeded attention towards myself.  For the most part, I can still get away with wearing medium t-shirts so long as I am layered up.  It is interesting now since it is my chest area that is beginning to be a bit more taught.