I piece with whatever thread I can find that works (doesn't show through) for the colors of fabric I am piecing - trying to use up older poly threads from making clothes. But when it comes time to quilt, it's only 100% cotton as my 6600 doesn't like anything else for FMQ. I buy my cotton thread from Connecting Threads. I love the wide variety of colors and even better when it is on sale! It is a bit linty, but a more frequent cleaning of the bobbin case takes care of that.