The objective is to keep the center approximately at the same height as your head, similar to your mains, but this configuration rarely is possible in most setups. Mounting the center just above the screen or just below out of the screen's viewing area should yield similar results. Angle the speaker down or up towards the center of the listening area, depending upon whether it's mounted above or below the screen.
Purists will gripe that timing and timbre will change due to the location of the speaker and angling of the center to the listening sweet spot, but unless you're a 'Golden Ear' audiophile, you'll likely never know the difference.
Might as well mention I have my center in my dedicated HT mounted on top of my widescreen monitor slightly angled down and have a center in my secondary HT setup mounted just below the screen and dialog as well as all else handled by the center sounds just fine... -TD
