I can't speak for MeanGene, but I have tweeked my center channel at times as well. I have a post regarding center channel being overpowered by mains in some movie passages. It was either changing, adjusting the trim on mains down, or up on the center. Since I listen to a lot of music,
I make tweeks on the center, by ear for different movies if I feel it is needed.

There are no rules set in stone that I know of, it is, as always, a matter of preferrence and how you hear the source.