The way to get the best sound of your center speaker is to aim the drivers at the listeners. An easy and cheap way to do this is to use a couple of rubber doorstops from Home Depot under the back of it to aim it down, or if it's under the TV, a couple under the front to aim it up (though, I would think the first option would win for aesthetics). Play with its placement and see how you get the best sound.
As for relations to the main L&R speakers, all three should be about on the same plane, as poorly illustrated with test below:
{L}____________________{C}_____________________{R}
<-------------------------PLANE--------------------------->