The only thing that you would compromise tuning a speaker meant for L/R use on it's side would be that of dispersion. The center speaker's dispersion is optimised for sideways mounting. They usually have the d'appolito arrangement where the tweeter is in the center, between two "woofers". Now, if you can find a speaker intended for main L/R use that has this driver arrangement, (or has a single "co-axial" driver) this might not be as much an issue, but you would have to talk with the manufacturer to see if there are any other considerations specific to their speaker. Of course you can always try putting one of your current L/R speakers on it's side and see if the dispersion suffers, and if it sounds good to you, then you may be able to use one that way.