Normally covering material is fabricated in a certain way. Rolled, extruded, or something. The bottom line is that there may be a "grain" to the material. What this typically means is that when shrinking the covering, it will shrink more in one direction than another. Try it for yourself. Get a scrap piece, draw a circle on it, shrink it. Is it a circle now, or an oval?

When covering open bays in a wing, it sometimes helps to have the grain run the length of the wing. This is because the wing surface is curved from LE to TE, but flat from root to tip. When shrinking the material it will pull tighter between the ribs than from the LE side of the bay to the TE side of the bay. The shrinking from the TE to LE sides of the bay will pull the covering down into the bay a bit (because the covering wants to take the straight-line from the TE to LE and not follow the curve). If you run the grain from root to tip the "sagging" will be less than if you run the grain from LE to TE.

gus