Hi shoeless and all,
I tend to recommend against upside-down mounting only because no-one at Axiom has ever done blind listening tests to standard vs. inverted speakers. I'm sure it will affect the soundstage and the spectral/tonal balance, but to what degree I just don't know. Of course you could try them out inverted and if they sound great to you that way, then fine. All our speakers are designed, measured and listened-to in standard non-inverted orientation--that's how they'll optimally perform.
Besides, the midranges/tweeters have effective enough off-axis dispersion that the soundstage blend with the center aimed down should merge effectively. Problems arise if the center is more than 2 feet above the top of the screen, which is why the over/under placement of two centers is the most effective of all.
Axiom Resident Expert