Hi cvictorg,
The center channel speaker does not have to be exactly at ear level. The dispersion of good cone drivers and tweeters is generous enough that the center could be up to 30 degrees off axis and you'd still get good intelligibility and frequency response. Axiom's stand for the center places the center approximately at ear level for a seated viewer/listener.
I would not advise putting a center on the floor because boundary interactions with the floor could cause some tonal colorations and annoying reflections.
Regards,
Alan
