If my borders are plain not pieced, I do sew them together before adding them to the quilt. I leave a little extra (about 1/2 inch) but pin to match the measurement and ease in the top. I try to get them all sewn on without having to add a little so I don't get wavy borders but I would rather be 1/4 inch extra then be short. If I am mitering, I leave the width of the borders plus the 1/2 inch extra.