I have done it; very, very rarely. I cannot remember the last neg I flipped. Of course, when my vision for the print was perfect the first time, who needs manipulation in the darkroom.....(Very Big Grin)
It's your photograph. Print it any way you damn-well please.
(Unless you live in a Communist country. ;) ;) ;) )
Sometimes as artistic license. But never if there is signage or any writing in the shot. If it makes a print composition better, why not?