Tegaderm is awesome and have used it on the last 4 of my tattoos. I did not use it on my back piece. I did however sleep on my back and I don't personally think it made any difference. I can't sleep on my stomach and told the tattooer straight up that I was going to sleep on my back. He didn't think it would be an issue and it never seemed to be. That being said, lay still and try not to move around when sleeping. Easier said than done i guess.
But if you don't have any adhesive allergies, I can not recommend tegaderm strongly enough. The stuff is a total game changer.