This question was asked in an acoustic lecture once. The answer was that center should be on the same horizontal plane as the left/right. In other words, the center midrange/tweeter should be the same height as the left/right speakers. This provides a more cohesive transition when sounds pan from left to center to right. It is even okay if all 3 speakers are mounted up high. It is still better than having a triangle setup (i.e. low main and high center). I remember seeing all sorts of luxury home theater installations that look great, but have the left/right main speakers mounted down very low and center mounted up high above the big screen tv.