I learned this many years ago when covering the two halves of a trainer wing. I had not planned very well and had cut the pieces for the tops of the wings oriented two different ways. I didn't notice the difference when I was covering, but when I had finished both sides I was shocked at how much difference it made. To make sure this is really what caused it, I purposely repeated the same thing on another wing. It did exactly the same thing! The amount of sag between the rib bays on one side was definitely different on the other side. You guys that are doubting this probably have not cut the material for your sufaces with it oriented 90 degrees off before, because when covering a wing you usually lay the material out oriented the same way as the wing before cutting it. I think that's what happened to me. I had covered several wings before that and never noticed a problem, but after making the mistake one time (and once more for the experiment) I have been very careful to avoid it since then. By the way, it seems that the softer, lower heat coverings exhibit more of this behavior than high temp coverings such as monokote. I decided this after thinking about my own experience and talking to friends who had, at one time or another, had the same problem.