I was once at a Nikon Desk to try my lens on other cameras. While replacing it I put it on the desk face down - front element down. The man behind the desk, without a word, took it and reversed it - bayonet down.
I didn't ask but I can imagine why now: some lenses have the front that moves, zoom or focus or both. Putting the lens face down may force some mechanism, which on the long way may result in a non uniform movement.
I guess this can be even worse during transport.