I do recommend 2 things I think are important for anyone rebuilding with cotton.
One is boiling the cotton
None of the cottons we buy were made with vaping in mind so will have residual chemicals from the bleaching process and others that make the material more pliant. I have tried vaping off of non boiled cotton and boiled and boiling does give it a much more absolute clean vape that is noticeable. Also usp means an acceptable purity of a product and is approved for its intended use, for instance some cotton gauze usp may contain antibacterial chemicals we might not want to vape. So I highly recommend boiling any cotton you are going to use.
Here is a site that goes into detail about the different chemicals in cotton products by another member.
MyWikkaSite: Cotton Wick Wiki
Two would be always preform a burn test:
This is especially important with cotton twines, yarns, and rope because some companies use a polyester or nylon blend to make it easier to hold the cotton fibers together. I have bought twines that claim 100% cotton on the label to find a burn test confirms synthetic fibers.
How to preform a burn test:
Take a small 3 inch piece of the cotton in question and slowly approach the flame (lighter/candle). As the cotton approaches the flame it shouldn't melt or recoil from the flame. Pure cotton should burn easily with a yellow flame. One of the biggest tell tale signs of its purity is when you blow the flame out the smoke should smell like burnt paper (synthetic fibers will smell plastic like) and the black burnt part should feel soft if pure cotton, if it feels hard and crispy it's an indication of synthetic fibers.