I Have had this happen too. Starching should help. Strips that are cut the width of the fabric have more stretch than strips that are cut the length of fabric. If you stretch the fabric to fit when sewing two strips together, they will pucker. I do not always have enough fabric to cut my boarders on the length of grain, but when I have enough , I cut then that way. I measure the quilt in three places, cut the average length and cut the boarder strips. This gives me a boarder without puckers. Sashing strips are another matter, I find that they are best cut the width of the fabric.