One of my evening dresses, only one (a long halter top) will show a little cleavage but not excessively. I'm not very big normally but I still have the extra weight on them from the baby so, oh well, may as well make the most of it before they get smaller!
I agree that there is a difference between sexy and slutty though although a lot of it has to do with confidence. If you feel confident showing cleavage it will look fine. The problem is when people feel to shy once the outfit is on and their body langage tells the tale.
A couple of my swimming costumes are possibly a little revealing in terms of cleavage but okay, they are bathing suits after all!