Having sewn for 50 years now, oh my has it been that long, well anyway I have a ton of odds and ends of sewing thread. Some of these spools I have acquired by those that quit sewing, but most are mine. I know they will rot if not used up. My son isn't into sewing and my granddaughter thinks she wants to, but then she is into playing volleyball and baseball. So I am not holding my breath over that one. She is who she is and I can't force her into sewing if she doesn't want to.
So to get back to the tip[. I am going to start using the more neutral colors on my bobbins and use them up in piecing. No it isn't all cotton, but you know I made clothing all those years and it didn't matter if it was cotton or polyester back then. I will use 100% cotton on the machine quilting though.
I even have some hand quilting thread that is poly cotton wrapped. So they never made a quilt law that said it all had to be cotton. I know I have heard the story about the poly cutting the threads of the fabric and yet none of the quilts I am making are going to be here in the year 2112. Do you know what I mean? Mine get used everyday and the issue of being a true quilter is the thrill of the challenge and the results for your purpose.
You use polyester quilt batting and poly and cotton batting and that never seems to matter. The quilt police will not be on my door steps at least not in my life time. I am using up the extra thread. Chris