Any reason not to place stand mounted speakers upside down?

It really depends upon the specific loudspeaker.  Some will have problems, as others have noted, and some won't.  Same thing with turning them sideways.

Agree it w/onhwy61 depends on the design.

For example, you don’t want to turn a time aligned speaker on its head.

But sometimes depending on your speaker height vs seating position it could be desirable.

If your current Speaker placement is at a Null in the room then a Boost in 
Bass would be good ...

Lak has given you the best advice. I have the HL5+ and I use Skylan stands that are specifically made for my Harbeths. However, I had them build mine 2" shorter because of my somewhat low seating position. Perfect!

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