Depends on you room response. You can get a sound pressure level meter cheap from Radio Shack, set it up on a tripod put in a test cd with warble tones and measure at what frequencies your room amplifies the sound. These would be the ones to tame.
Bass Trap Questions....
I am in the understanding that the diameter of a bass trap can affect the kind of frequencies that will be trapped, but how does height matter? Obviously, having a bass trap from floor to ceiling would be most effective, but is only having a four foot tall one at each corner enough to do the trick? For two channel music, what frequencies should be trapped?