I've always used size 50 for piecing and heavier for hand quilting. Machine quilting with heavier thread causes me to have to fill many more bobbins.... the extra thickness takes up more room. As long as I use the same thread on top and bobbin it's fine. If you mix sizes, the tension will get messed up. I have lots of thread..... I find thread stashes at auctions, estate sales, and yard sales. I collect the wooden spools and if the thread is still strong, I use it. I have lots of colors for applique that way.