I feel that the tension is actually pretty close to 'right on' with just a peek of the other side where they meet on the straight stitches. The thread that is pulled to the other side on the corners is actually the bobbin thread drawing the top thread to the underside. Corners are tricky and so trying to get the needle in the correct position to pivot so that they are square on both sides is where the 'problem' is coming in.
I thought Archaic Arcane did a post about tension and stitches that was very good, but I can't find the one about the stitches right now.
Janey - Neat people never make the exciting discoveries I do.