I’m going to assume for argument’s sake that it’s your choice if you’re a gentleman and want to flaunt your moobs or your coccyx. However, the zeroth rule of cleavage, before anything you might see in glossy magazines, is any cleavage you show must be part of your own body.
I’ve seen it said that you should only wear ties made of a material you’d willingly wear a shirt in. As we have seen from the European Space Agency, this guidance is too restrictive for many people and not restrictive enough for some. However we can all agree that nobody likes a novelty tie. I therefore take the converse: only wear shirts with patterns you’d willingly wear a tie in.
I hope these rules I have adumbrated are helpful.