[QUOTE=Bobbinalong;8299885]For a diagonal join you will lose a little more than the width of your binding strip.
You will also lose a little binding with the mitres on the corners. Quote]
No you don't lose anything on the corners. The edge of the binding lays on the edge of the quilt and just goes around the corner. There is no fold on the exact corner.
The reason for sewing binding on the diagonal is to reduce bulk. If you make seams straight across, you will have 12 thickness of fabric at that spot, plus the quilt top, back and batt. And that's if you pressed the seam open.
But I don't cut my bindings on the 45º either. I lay the strips on the mat, lined up carefully with the printed lines with one inch of overlap. Then I cut an angle from the corner of the top one to the corner of the bottom one. You can't see the bottom one, but the lines on the mat will tell you where that is. That way I only lose one inch instead of the complete width of the binding.