If you are quilting yourself, then cut it to suit yourself.
HOWEVER, if you are having someone with a long arm quilt the top for you, they will need between 4 and 5 inches of backing fabric on EACH OF THE FOUR SIDES OF THE QUILT TOP. This allows for room to check tension, and for starting and stopping off the top so there are no "empty spaces" on the edges of the top. I personally ask for 5 inches on each side or 10 inches wider and 10 inches longer than the top.