I do not quilt for money----just myself and charity. If possible I always pin the top and bottom of the backing using the selvages. I have ripped down the sides. I then roll is a bit so the batting and top are a inch or so down from the selvage. Sometimes it is little wonky when rolling just the backing and if you roll it back an forth a bit it will straighten itself out.
I do not prewash the backing. I have no idea if a professional would approve of this method but it works great for me.