I have pieced border with 15 degree angle and also straight. I like the diagonal seam, but takes much more fabric.
I usually start my first borders with 2 width of fabric cuts. That will go on top and bottom of most of the quilts I make. Then by cutting only half the width I've cut across my fabric...20+ inches, with two full widths and the half will be enough for the length of the quilt so I don't end up with a very small piece at one end.