I've use cotton, rayon, polyester, polyester wrapped cotton and monofilament threads in my quilting, depending on what I'm doing at the time, and if the threads match the project. If I don't want my top stitching to show in quilting, I may use a monofilament thread, which is nothing like the monofilament thread that was around when I first started quilting, which was a little like fish line. In the last several years I have done my quilting with rayon thread, mainly because I like the look, as well as the little extra zing it gives my quilts. It is your quilt, you can do whatever you want. The one rule I honor in quilting is that there are no rules.