My DH taught me how to square up a quilt as you sandwich it. Just because the sides are of equal length does not mean that the top is laid out 'square on the batting and backing. Once you have the top laid out and smoothed to your satisfaction, take a tape measure and measure DIAGONALLY. If those two measurements are the same or at least VERY close, your quilt top is square.
DH uses this method in woodworking. I am constantly surprised at the similarities of techniques in working with wood and fabric.