Consistency is very important, meaning you have to mount the TV in relation to the average watching position. Do you sit down most of the time or do you entertain a lot and people watching the screen are always standing up?
If you watch TV while sitting on the couch or lazy boy, the flat panel should be mounted at eye level to the center of the screen from your height sitting straight on the couch.
Problem is, this could make the TV seem low when standing next to it. So a compromise is in order, adjust up so the bottom of the screen is eye level while sitting and the top is eye level when standing.