In the unlikely event of the center channel (not the subwoofer) creating a problematic low frequency resonance, moving it six to eight inches wouldn't be enough I'm afraid. You'd have to move it more to one side, which would mess up the symmetry of your front sound stage. The dialog wouldn't appear to come from the right place, among other problems.
If your center speaker actually did succeed in exciting a widthwise resonant mode, it would likely be fairly high - over 100 Hz. This is best controlled through wide-band absorption, which a home theater probably needs anyway to maintain uniformly low reverberation across all frequencies.