Here we'll collect the various sound advice from audio/video publications relevant to the center channel. In a world of no guarantees, you can always count on these pointers:

Home Theater

"Set up the speakers so that their drivers are aligned vertically. This will provide the smoothest off-axis response. Horizontal center speakers are an exception, but this arrangement can compromise the performance. The horizontal driver configuration in most center speakers is driven by aesthetic and marketing considerations, not the best sonic performance."  Speakers: Where Do I Put Them?, Darryl Wilkinson, Feb 2010

"Horizontal dedicated centers are a throwback to the days when bulky TVs ruled the roost.  Their persistence in the market is rooted in the popular misconception that only a horizontal speaker can serve in the center position.  The people who design and market them know better.  Dedicated centers usually have a left-to-right woofer-tweeter-woofer driver array that's prone to horizontal lobing.  Lobing is the result of closely spaced drivers' propensity to sum and cancel each other, which causes uneven response from seat to seat around the room.  Better center speakers mitigate the problem with a vertically stacked midrange and tweeter or different crossover points for the individual woofers.  When a woofer-tweeter-woofer speaker operates vertically, what had been undesirable lobing can be seen as a possibly desirable limiting of vertical dispersion, which can reduce ceiling and floor reflections.  The best center speakers duplicate the sonic characteristics of your front left and right speakers.  An exact duplicate, placed with the same orientation, works especially well."  Loudspeakers Buyer's Guide, Don't Underinvest in Your Surround System, Mark Fleischmann, Feb 2010