I use a thin strip of fusible web slightly smaller in width than the seam allowance on the miter. Then iron I to the seam allowance only , then iron it to sandwich. It keeps it from moving around and getting stretched. If it a particularly wide border I sometime use a very thin strip( 1/4 inch) of fusible at the outer edge for about one foot on each side of the miter to really control the any stretch in that area.