I like doing a pieced strip or two in my backings if I don't have a large enough piece to do the whole back. Sometimes parallel, sometimes not. Depends on my mood and the "mood" of the quilt. I guess I figure if I'm already putting seams in the backing I might as well make it interesting!
I did a OBW for my sister out of panels and I left one panel intact on the back so she could see the source and just put really wide borders around it. I think that is my favorite quilt to date, both front and back.