If I've got it zipped and the hood cinched, I roll the bag with me. If it's warmer, and I don't need to batten down, I just unzip it all the way and lay it over me like a quilt; my Thermarest pad keeps me insulated from the ground in those situations. If I'm using it as a quilt, I turn underneath it.

It all depends on how cold it is versus my bag's rating.

I've never had any problem with my back getting cold when I roll the bag with me onto my side.