I color match, often use polyester and frequently use what's left of the thread on the bobbin of my long-arm when I finish a quilt. Polyester doesn't wear out cotton fabric. I just believe that's an urban myth. So, for the sake of invisibility, fine and color matched are my checkpoints. The thread also needs to be strong. You don't want any breakage here and the binding of a quilt takes more stress than any other part.