OK, picture your newest wing. It's built up with open bays right? If you cover it with the roll direction inline with the chord, the covering will dip more between the ribs than if the roll direction were inline with the span. There are no wrinikles or difficulty in getting the covering the shrink, it's all about how much the covering dips between the ribs. The more dip, the less accurate the airfoil and the less aesthetically pleasing the covering job.

Again, I will post pictures this evening of the top and bottom of a wing with a ruler across the ribs to demonstrate the difference this makes.

John