Assuming we're talking about a corporate setting and not a bar...I think there should be no cleavage showing. Seriously.
I like...no I LOVE boobies as much as the next guy...and I mean all types of boobies. Big, small, perky, saggy...fun bags galore. But not at work.
If you show boobs, you're trying to get attention/hit on/promoted.
If I look at the boobs you're showing, then I'm a pig, a male chauvanist, a horn dogger, a serial rapist. It's a no-win for all involved.
I'm not saying to dress like a nun. Heck, a nice knee-length (or slightly above) skirt and a shirt with the top button (maybe two) undone. Collarbones are nice.
But don't show me your boobies b/c then it affects The Business. And we dont' want that.