I prefer a straight join. The time I tried a miter on a wider border, it was harder to get it to lay perfectly flat. I don't think a well pressed seam shows very much after it's quilted. I actually like a border cut crosswise better than one cut lengthwise. The little bit of 'give' in the crosswise helps 'fudge' discrepancies in the lengths of the sides of the quilt so it stays square (or rectangle).