Any reason not to place stand mounted speakers upside down?

I think teo audio mentions the important point.  The pattern of vertical dispersion is generally something like a wide fan, with a 90 degree width and a 10 or 15 degree height.  The fan is aimed slightly downward from the tweeter.  Imagine a cross section of this pattern that looks like a super narrow triangle, the tip of the triangle is the tweeter, and one of the long sides is parallel to the floor and the other long side is angled down by 10 degrees or 15 degrees from level.  So when the speaker is vertically oriented with the tweeter on top, the pattern is just a bit below the tweeter; flip the speaker and the pattern will reverse, it will be ever so slightly upward. So now you need to position yourself slightly above the tweeter.  The worst is when you rotate the speaker and set it on its side.   Now its a a narrow fan offering 10-15 degrees of width and a very wide vertically, splashing HF on the ceiling and floor.   Try it at home, its dramatic how a speaker on its side means that you move just a little bit left or right off axis darn near makes the tweeter almost go away.  This is a good demonstration in your house of HF pattern.  This works with almost every kind of conventional two way or more cone loudspeaker.
Brad

Id say close to zero [conventional 2way/3 way] speakers designed to be vertical can work properly sideways (rotated 90 degrees or horizontal).  The reverse is also true, very few designed to work horizontal can work vertical.  You must be aware of the speaker dispersion pattern and the designed orientation.  Think about headlights in your car that are designed to be wide and not "tall"-same idea.  Waveguides and driver interaction at crossover (when they are both playing the same thing) determines this dispersion pattern and tweeters usually need to be physically directly above the driver it crosses over to.  There are some exceptions, but few.  Upside down is at least vertical,  so you can at least figure out someway to make that work- most of the time.  But beware of energy hitting the ceiling just as you would energy hitting the floor in the "right side up" orientation (why rugs make so much difference).  Reflected energy is usually the source of most problems with speaker/room interaction and severely affect speaker imaging.  

Brad