I used to sleep on my side or my back a lot a few years ago, but lately, I've taken to sleeping on my stomach. My arms rarely get numb, if ever. Its usually the fingers that lose their circulation.
And as a side note, I roll, toss, and turn a lot. I've woken up on the floor countless times, with my feet facing the opposite direction it did when I went to bed, or curled up in a ball... Its insane.