When I quilt on my sewing machine, I leave about 2 to 3 inches. I always start in the middle and work outwards. It doesn't draw up very much that way and the back is nice and smooth.
On the other hand, if you send your quilt top to be quilted at a shop, they need that extra to pin onto the leaders to roll the quilt to sandwich it. Also on the sides so they can pull it taught with straps so when it is quilted, it is nice and smooth and pucker free.