It is my understanding that it should be mounted so that the center of the screen is at eye level when seated.
When lying in bed your eye level would be a bit higher so you would adjust for how you and or spouse lay and at what angle you would be sitting up to watch TV.
Most people mount TVs way too high...usually above fireplaces, etc., you now have to tilt your neck to watch... like your in a bar, many retail establishments have highly mounted TVs only because that's the only place to put them, not necessarily cause it's a good viewing height.
It's best to use an articulating mount so you can adjust for different viewing angles...a fixed wall mount limits your viewing angles and thus you don't get the best picture from varied angles.