It depends on the quilt. If the border fabrics are a light solid, I try not to piece them as I think it is to obvious. In a busy border fabric you can hardly notice a join especially if you quilt along the seam line. I sometimes place a block into the border or add a design to diguise a join. Sometimes I miter the corners, sometimes I do a straight corner and sometimes I add corner blocks. You will need to decide what is right with your quilt pattern.