No,they are not accurate!! When I was manager of a JoAnn store in 1973 these were installed, as well as the timed locks on the doors. Neither one of these items were accurate.
I was always in the store by seven but didn't open until nine. I would unlock and then relock the door. Being sometimes forgetful I wouldn't set the lock until 10 or later. The home office looked into this, I told them the reason, a few months later these locks were removed from all stores. Big money saver! Back to the fabric... Satin-type fabric would slide through too quickly and wools too slowly, therefore you could make out like a bandit or get cheated. Granted it did work on some fabrics.