The site says, Tips and Techniques. So here's a tip for building straight wings.

Locate the tip and centerline rIbs, and mark a centerline from the center of the leading edges to the center of the trailing edges....similar to plotting for cutting a foam wing. Now glue a 3/8"X3/8"X3" balsa stick below or along the line. The stick provides a shelf on which you can place a small plastic line level. Now you have a way of determining when the wing panel is exactly straight. As long as the root and tip rib bubbles agree, the wing panel will be straight.

After a lifetime of modeling, I've built my share of crooked wings. Using this method gives me absolutely no excuse for building-in a warp. And, although it sounds a bit more trouble, I think it's actually a little faster as a wing can be built with very few pins....using a little spray glue to keep things in position in lieu of pins. Wings usually require only about a half dozen pins and a couple shim sticks to build.