I believe it is a sin to show cleavage.. when you wear a dress that shows cleavage knowingly, you are obviously committing a sin because it obviously temps men! If you do it unknowingly or do it because your dress "feels right" or you "do it for your husband", you are neglecting God! Yes, you are neglecting his commandments! Because you are still showing cleavage that may tempt some man! Say you are walking through the crowded streets of Manhattan showing cleavage, even if it is only little.. you are only considering what's good for you or what makes you feel right.. Jesus doesn't want us to do that.. you have to think about all the men out there that might look at you lustfully and cause them to sin! If Jesus only thought about himself, then we would not have eternal life or find favor in God's eyes! So do what Jesus did, live for others.. live for the comfort of others!
Also, if you look at many of the women in other cultures around the world, they dress in a conservative, non-revealing way! I believe we should have the freedom to wear what we want, as long as it doesn't offend God's commandments!
Have a great day and God bless you all!