I was wondering if it'd be a good idea to wear tops that show a little cleavage with a flat chest. Probably not, but worth asking.

Why not?  I do.  I guess it depends on the person and their situation.

As long as it's appropriate to the situation, I don't see why not. Offices mostly frown upon cleavage, but if you're going to a situation where you won't be judged as a professional, go for it.

Some of us find flat chests extremely attractive

You'll only pull it off if a push up bra forms a little bit of a cleavage.