Most LAers ask for 4 inches on all four sides. I have quilted one that the backing was flush with the top. It was for a friend and It was the first time that she had one quilted.. I added a 2inch strip to the top of the backing and was able to quilt it very close to the edge. Good thing it was a small quilt and she was happy with how it turned out. She learned that we needed some extra fabric to pin the quilt on the leaders.
I don't really need 4 inches, 2 inches are enough for me if the backings are blocked but sometimes the backing are not blocked and it that case in order I have to straighten them up and that means needing more fabric to work with on either side.