As Prism99 says, it depends on the type of sewing. I like to use mostly polyester top and bottom, as it reduces the fluff that gathers around the needle and in the bobbin areas. When using a thicker cotton on top, I find with piecing, if the darker fabric is on the bottom, the seam allowance will fold better to the dark side with not so much bulk. I do like the pre-wound bobbins, but find when half used, the top thread will not pick up the bobbin thread after using the auto. cutting button. I think this is because they are wound so tight the thread shrinks back a little, so I re-wind the last half on my bobbin winder - it works. Except for the embroidery machine, I do use normal cotton thread for many things, it's just a matter of testing on scraps to see the difference, to see what you like.