When you line up drivers on a speaker, you get cancellation in the direction that the drivers are aligned and wide dispersion perpendicular to that.
So, if you place a typical speaker sideways, you'll get cancellation horizontally off axis and wide vertical dispersion. If you're sitting directly in front of the speaker, you'll hear it fine. But anyone sitting to either side of you on the couch will not hear it so well. Not good. The wide vertical dispersion means that you'll get reflections off the floor and ceiling, muddying up the sound. Not good. So it's a lose-lose situation.
Placing the same speaker upright will give you cancellations vertically and wide dispersion horizontially. That means that everyone spread out on the couch will hear the dialogue almost as well as the person sitting in the sweet spot. This is good. The narrow vertical dispersion means that you'll get less reflections off the floor and ceiling, making the overall sound clearer. This is also good. A win-win situation.
How you mount your centre speaker is up to you and, unfortunately, based on constraints you have to take into account. However, based on the above, I hope you can understand why Eric, Kal and others are urging you to place it vertically (if you can).