If your border fabric has a pattern to it, you might want to adjust to make the pattern in the mitre join in a pleasing way. I usually lay the border fabric along the side and top and look how the corners would match. If matching requires longer strip then I will take a few tucks down the side to bring it into place. I believe one looks at the mitre first if a printed fabric and is better to have some kind of logical match at these points. If the fabric is directional which is usually the case I might split at the half way point on sides and ends and take up the difference in these places.