quite simply, monitors should be placed on their sides if they sound better that way. that's more room and position dependent than design dependent, but check with the manufacturer to see what they recommend.
if your next question is, "should the tweeters be on the inside or the outside?", then, again, it depends on what sounds better (though i do believe tweeter-outside is more common). just make sure the monitors are placed to be mirror reflections of each other.