Like everyone else here I have used just about averything and anything. I just completed a quilt this past weekend and I used extra thick fleese as the backing for added warmth. I have a topper on the frame now that I purchsed in a sale and it had a lot of rough spots and I knew I would never be able to sell it, so I am using it to practive using the pattern perfect templates and I am just using a sheet as the backing on it.
I have also for batting used old fleese blankets that no one in the house wanted anymore, I just use a lint shaver and get all the fuzzies off them and sandwich them right on in there. So the old blanke gets a second life that way.
Use whatever you are comfortable with.