In order to keep your quilt square, you should measure down the middle of the quilt, either horizontal or vertical, cut two strips that measurement and sew, either the sides or top and bottom, easing the strip to fit if need be. Then measure down the middle again, including the strips just sewn on a repeat the process. This will avoid "wavy borders" which are the bane of any longarmer. You want your quilt to lie flat and be square, just sewing a strip to the edges will distort the quilt. I fixed one quilt for a lady who did this, and she had 3 extra inches on one side of her quilt, and it was only a 40 x 40! I hope this makes sense to you.
Michelle G. in El Paso