Different people use different thread for different reasons. It probably doesn't matter all that much, it's what you are happy with. I was told in the beginning (a couple years ago0 that a good quality cotton thread was the best, so that's what I use. On wall hangings & things that aren't going to get laundered much, I use what I have. I have more colors in poly than I do in cotton, so that is easier. For a larger quilt, I'll purchase the thread I need (cotton). Sometimes I've used invisible thread, as my FMQ is not that great yet & the invisible thread covers a lot of mistakes....:-)