You should know looking up at the TV with your head tilted up will cause more eyestrain than if you're looking straight on at it.

Having the TV below your eye level, so your head is tilted down, also produces less eyestrain than if you're looking up at it. The same type of thing applies for computer monitors, so it's generally better to keep your monitor straight on or below your eye-level.
That being said, eyestrain is nothing serious and at worst might give you a headache.