I have seen numerous problems caused by vibrating things not meant to be vibrated. Moire patterns, tubes gone bad, cables working loose, internal connections breaking, you name it. There are many variables, of course, since the amount and sensitivity to vibration comes into play for all the various components, but I prefer to limit direct coupling if I can.
Most speakers disperse more in one plane, like horizontal, than the other, like vertical. Below a certain frequency all speakers look pretty much likea point source, and they become more directional as frequency rises. A speaker designed to be oriented one direction may not sound the same in the other direction, especially off-axis. Personally, I suspect (but do not know) laying a "regular" speaker on its side would work OK for the center since now it is most likely more "vertical" in spreading (dispersing) the sound and thus less likely to interfere with the L/R speakers. One could make any number of pro/con arguments, of course. You could try both ways and see what works best for you.
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley