You are correct, nylon works fine. Most rig companies have plenty of parapack that happens to match your rig perfectly, it hot cuts well and is easy to work with. However cotton is a much better choice for deployment bags, not only for CReW but anytime you are deploying a canopy out of a bag. The nylon on nylon friction causes burns that weaken the fabric, and a fight between 7 oz/yd parapack and 1.35 oz/yd ripstop would be an easy bet to win. Cotton bags are harder to make since, among other things, cotton does not sear when the edges are cut, you have to apply an anti-frey solution to the edges.
The bottom line is that most folks don't give a rats ass, but if you love your canopy like only a CReWdog can, get the cotton bag. Your canopy will land you softer because you cared.