I have always mounted "bookshelf" speakers upside-down when they were actually installed on a shelf to get them better aligned with ear level. The speakers generally had "early" dome tweeters which probably didn't have as wide dispersion as the ones Axiom uses today.
Note that the definition of "bookshelf" back then included some fairly large speakers
My recollection was that the sound started to degrade when my ears were maybe 15-20 degrees below the tweeter centerline. These were all two-way speakers so I can't comment on Alan's statement re: the midrange actually being more important than the tweeter, but based on the range of frequencies that seemed to be improved by inverting the speakers "back in the day" that statement (worry about the midrange) makes sense to me.
