Looking at Google's images for a Patrol Shelter, it appears that you will need a catenary cut for the roof seam. So it's the top edges that I think you will have to cut slightly across the grain to get the arc of the catenary cut. You can orient the material so the bottom edges are with the grain.
In woven fabrics, there are a warp line and a weft or fill line, perpendicular to each other, in the grain. One runs in the long direction of the role, the other across the roll, and frankly, I can never remember which is which, except that on some fabrics, the one that runs in the long direction is stronger. And I've no idea if that is the case with Cuben (except with their bias layups, that run in more than two directions). I doubt that this will be an issue, as the material was originally designed for sails that need to be strong in both directions.
As for stretch, Cuben doesn't stretch very much in any direction, unlike woven nylon that stretches in all directions, and woven polyester that stretches on the bias.
It has been said that with heat and prolonged use, Cuben will permanently stretch, as when a sail develops a permanent ballooned curvature. Just by manipulating the material, I have seen that the mylar outers (not the dyneema fibers) will stretch a little when warm, but it does not return, right away anyway, to its original shape; that is, it is not elastic. But, this doesn't seem to have been an issue in small tarps reported on here on BPL.
Hope you have read a lot of the stuff here on catenary cuts and tie-outs, as it is essential to know about for a durable product, and Cuben is expensive.