I use whatever I have on hand! There is always going to be a million different opinions about thread just like there is about fabrics, sewing machines, pressing, pre washing, etc. You get the idea! I use Coats & Clark Dual Duty XP which for years I thought was 100% cotton but not too long ago I looked at the label and it is 100% polyester covered polyester! I know that there are concerns about polyester threads "slicing" cotton material but I really don't believe that will happen in my lifetime! I have used Gutterman Rayon for decorative stitching or appliqueing, and Signature which is mercenized, whatever that is. I have used 100% cotton too. Usually it is whatever I have and matches what I am sewing.