Where do I place my center channel speaker?

I am forced to have my 6 speakers (2 front wide, and 4 surround) near the top of the wall but all are pointed at my seating area. I am able to have my center channel anywhere from right above my TV or up level with all of my speakers. Should the center be placed as low as possible? Or as level as possible with the other speakers? The center channel is being placed on a flat shelf so I will not be able to level it at all. Center channel speaker is the Polk Audio CS2 and my surround speakers are Acoustech PL-66's.