You can get a bad miter saw. Several years ago i bought a 12" Delta compound miter saw. I could not get a tight miter joint to save my neck, and this was on door casing when I was running trim. I exchanged it for a 12" Dewalt and problem cured.[/b]

That was the problem. You bought a Delta. It wasn't that particulat saw; it's the whole crappy line now. The exception is the AMERICAN made industrial tools. But even those aren't what they used to be.

These are crap.
http://www.deltamachinery.com/index.asp?e=136&p=874
http://www.deltamachinery.com/index.asp?e=136&p=796

These are better.
http://www.deltamachinery.com/index.asp?e=136&p=4735

Ryobis are about the same quality as delta if not worse, Ive also found that if you try to recut a miter the blade will actually push the board out of the way a little if its not nice and sharp which results in a slight curve to your cut. The best edge for a miter is the first cut though solid material.

Another way to get though a 5 inch board with a 10 in blade is to put a waste board down on the saw table first and then cut the work piece, This places the workpiece higher up the blade diamiter and usually will cut through.