I use 3 when piecing. I always backstitch at the beginning and end of a stitch. I've never had a problem with the stitches being loose. I think it depends on how balanced your stitches are and the perfect tension to get balanced stitches. Not many know what a balanced stitch looks like. Machines are set to sew poly thread with poly bobbins. That is the auto/normal tension the machine is set for, even the bobbin tension. You have to change tension with every type of thread.