My understanding of ergonomics and its rules says that you should have the top of the monitor at eye level. Though, this becomes less true for larger displays. With a 22 inch display, I say you should follow the rule. If you flip a 22 inch display on its side, I'd say it should be about 1/3rd down. As you have some 23 inch monitors, you should come close to the rule. Maybe a bit closer to center, but direct center is deemed inappropriate by ergonomic standards.
If you check here, you'll find most images say the same thing. But I agree with the above in that you should seek out what's most comfortable. I personally think it's best to set up your desk and chair in a way that encourages you to have good ergonomics. So raising your monitors a teeny bit might make you straighten your back to meet the right height. But you're encouraging yourself to aim your head up and arch your neck should you begin to slouch, which just amplifies the effect.
Get comfortable. ;) Might I suggest a kneeling chair? :onethumb