Any reason not to place stand mounted speakers upside down?

Then wouldn’t it be in reverse polarity then? I guess you could stand on your head then it would be in correct polarity. 

There should be no problems with mounting a speaker upside down, unless the woofer magnet assembly is being supported by an internal brace that is relying on gravity.  That would be an unusal setup (most drivers are not supported internally.  Harbeths are not supported this way).  

You can certainly try inverting the speaker.  However, there might be some complications as far as the sound is concerned.  This might increase the problem with floor bounce cancellation of lower midrange frequencies.  The higher the woofer is off the floor, the greater the chance of floor bounce being an issue.  To some extent, designers try to account for floor bounce, but, they never account for someone inverting the speaker and increasing the height of the woofer.

Thanks @larryi 

Interesting point.