It depends on too many variables to give a singular answer.

I'd look at the overall load time and see if your current PNGs are substantial part of it. If not, I wouldn't worry about further optimizing them except for the bare minimum of throwing them through imageoptim.

If they do significantly contribute to your page load, then compress them to a point you're comfortable with. It's more of an art than an exact science. If you're an illustrator you might care more about pixel perfect images than an average blogger for example.