I have read that it is good to have some light on while watching televison. The light should be low level and to the rear of the set. The reason given was that this will help your eyes not to become fatigued during sudden changes in contrast and brightness levels that happen on the screen.
On the other hand if you are using a front projection unit, the darker the better.
I like a little light behind my set. This helps me not to trip over the dog during potty breaks.
Dave