If you're covering open bays with it, that might matter. It might not matter either. From experience, I'd suggest you buy a yard of whatever they sell the most of, and test it.

If you're covering sheeting, I wouldn't hesitate a second using whatever they have. I've never once looked for "pre-shrunk" and used a bunch of polyester cloth as a lighter "glass cloth". It's magic stuff. Polyester resin, epoxy, polyurethane paints, bunch of things work very well with it. I've never had a problem using it to strengthen and reinforce. And never bothered to specially select what I was buying. OK..... I gotta admit that I actually don't usually buy the stuff. My wife does that for me, or gives me fabric she has decided she doesn't like now that she's gotten it home. Women seem to decide about half what they bought isn't what they really wanted so she's a perfect supplier of the stuff.

BTW, one of the most popular attributes of polyester is that it doesn't shrink like cotton or silk or other natural fabrics. So the advice to use "pre-shrunk polyester" is strange right off the bat.

BTW, the silk you find in those fabric shops also does pretty good for reinforcing sheeting as well.