I would not recommend using your tv's sound and your center channel at the same time. You will definatly get some comb filtering and reduced audio clarity and level.

If you simply want to watch tv without your surround sound, run a coax to your television and simply use the internal speaker to watch your shows. Remember to flip back to your video input that your receiver uses when it's time for a movie. You'll also have to turn on the internal speaker for this, and as I stated above, turn it off again when you use your receiver!