It is very difficult to get a quilt perfectly square. Shifting during the sandwiching, basting and quilting process can cause the top to no longer match the back perfectly. You really should get some complimentary fabric to your back and sew a border on to your back giving you another 4 to 6 inches all the way around. If you do not, you may find that when you reach the outside edges of your quilt in the quilting process you will have no backing. Not only that but as you get to the edges of your quilt you have nothing to hang on to without a bit of excess backing. Finally, most battings are cut a bit larger so when you sandwich, if your back is not bigger and you have the batting covering your back, it is downright impossible to line those edges up perfectly without a clear line of sight.
Of course you could trim everything down once finished but that could impact the design of your top.
You want your backing stretched taunt for basting to avoid getting pleats and puckers which can easily happen as you machine or even hand quilt if it was not taunt in the sandwiching - basting process.