Setting a screen with your Back

I had a debate with one of my pick up players regarding screen/picks. I said that you must set the screen facing forward. meaning you set the screen facing the defender. He says that you can set a screen or a pick with your back to the defender.

Is it a legal pick/screen if you set it with your back/butt towards the defender?

Help!!
Thanks

To my knowledge there are no rules about setting a screen or pick with your back, however, I don't get the reason for doing it. There's a whole host of reasons that's a bad idea, but unless you have eyes in the back of your head, I'd advise against it.