This is a good question but i think depends on the persons age!there is a rule that says if you show your cleavage then cover up the bottom half and this works the other way too.. so if you show your legs cover up your cleavage and you cant go wrong!

Ah, therein lies a tale. It depends entirely on what you've got. If you are well-upholstered in that department, you really don't want to make a song and dance of your cleavage; if yours are merely the darling buds of May, then a plunge neck looks a treat. The basic rules are as follows:
1. If you're showing plenty of chest, keep your hem longer, otherwise you will look tramp not vamp.
2. Make sure your bra is doing its job; a cleavage can be hugely enhanced with the correct underwear. Choose carefully and get fitted properly.
3. Make sure your make-up doesn't stop at your chin, leaving your helpless cleavage to wobble like a limpid blancmange in the moonlight. Add a dab of tinted moisturiser
4. If you are crepe-y or mottled in the neck and bust area, best keep your cleavage under wraps. Show off your ankles or wrists instead (they can be equally alluring).
5. You can distract the eye from too-full a cleavage with a judiciously chosen necklace.