I've often noticed that the ones that SHOULD and COULD show a bit more usually don't and the ones that DO usually should NOT

I agree that is should be okay for whatever the person feels comfortable revealing. If we brought less attention to the whole cleavage taboo issue, there would be less issue with it. It is like anything that we classify as off-limits, it makes us want it all the more.
As long as it is revealed in a tasteful, sexy way, go for it! If it looks like you're trying out for porn magazines or the latest issue of Trailer Trash, keep it covered.
(I'm allowed to use the term Trailer Trash. I live in a double wide

)