I sew my border strips together at a 45 degree angle, as this is the way I was taught. The reasoning I was taught is that when the fabric is joined that way, it is harder to see. I don't know if it's true, since I've never joined the strips with a straight seam. When I sew the borders on, I try to sew the strips so that the joins are not directly across from each other, but that is just my personal preference.