Ok here are my thoughts....l have a spare bobbin case that I use when I want to or need to mess with my tension due to threads, yarns or ribbon going through it. I never touch my "main" case. Most of the newer machines have "self" adjusting tensions. If I was using an older machine I would let a pro get it perfect for everyday sewing and write down all the settings and then go back to those after doing adjustments for a special project. In home-ec in my day we would get our hands smacked with a ruler if we touched the tension disk knob!