They have a little stretch in the bias. Just enough to mess up your sewing if you don't concentrate. But it's not "stretch" fabric like lycra, by any means.
Cool, that's all I needed to hear! I'm planning to cobble bias-strips into the inner surface of my UQ so it can "stretch" and conform to the hammock under tension. The sil nylon I've used in the past has little to no give, since I suppose its fibers are glued together by the coating.