a good speaker was designed to be used the way the designer tells you.
This is a pretty strong point. If a speaker is well designed, it is run through many paces. Many things are considered in positioning tweeter and woofer. Of course, that makes assumptions about your listening position, but they factor that in.
The effects of upending a speaker are variable, because of the factors mentioned above and which speaker you're upending.
Listening is surely a good thing to try. Do you like it better? Damn the torpedos!
Using an analyzer such as Room EQ Wizard is another path toward understanding what is happening when they're upside down or even on their side.