Actually...

A panel of dacron will have its strength in only one direction (both ways), because of the way the fabric is weaved from fibers. One a square piece, the strength will either be up and down or left and right. Generally, on a long roll of cloth the strongest direction will be from side to side, not along the length. But it also comes with the strength lengthwise.

On our normally panelled Dacron main and jib sails, the seams for the panels are on the battens, meaning the cloth is rolled out side to side on the sail and sewed together panel-wise going up, giving the strongest part of the cloth to take the biggest loads, mainly up the leech and luff. Anytime the force goes out of alignment with the direction of strength in the cloth, it will stretch and ultimately deform. This way of making sails obviously can't account for the fact that almost no load on the sail will line directly with the strength of the cloth.

Radial cut jibs and mains are an attempt to line up the strength of the cloth with the loads on the sail. This is a case where the strength in the cloth is lengthwise. But again, for all the effort, the load paths will not always line up with the strength of the cloth, leading to stretching and deformation.

Other types of cloths used in panel type construction attempt to have strength in more than one direction, but ultimately will still not be able to exactly match load paths. Still, an improvement. Most of our mylar sails are radial cut fabric laminated with the mylar. Same issues apply ultimately, depending on the directional strength of the cloth used.

The modern laminate sails, place fibers in the sail to directly correspond to the load paths, then laminate it all together.

Sorry, couldn't resist, from a lecture on sail materials from our local North shop...