You can use the borders to help square a quilt. The middle measurement is used a lot but you can do averages. The idea is that pin and help ease or stretch a bit (pieced fabric can grow) and with the borders the same on each side will even your quilt. The same for the top and bottom.
Sometimes with smaller quilts or ones that are more casual, you can just add a border strip, sew and cut off. However, on longer quilts, doing this can create an 1” or more difference per side.