I've never pieced a border with a diagonal seam. If I have to seam it, I place that seam where the seams between blocks already make the eye see a straight line. Until 18 months ago I had never heard that piecing a seam on the diagonal was "right", and I've been quilting, teaching, designing, etc. for over 30 years. When someone tells me that, I just answer, "Well as I wasn't invited to the committee making that decision, I'll pass on that "rule" thank you."
Jan in VA