I keep at least 4-5 inches around mine if it is bed-sized; less if the quilt is smaller.
BUT, I turn that overhang and fold it to the front side as if I were going to use it as a binding, and safety pin it down. I found that leaving it loose allowed the soles of my walking shoes to drag against and tear the batting! This also helps me fold the quilt up better for when I'm transporting it or setting it aside for a while.
Jan in VA