The safest/smartest thing to do is buy a second bobbin case, imo. One that stays as the dealer adjusts and one that you use in cases when you want to adjust the lower tension yourself, for heavy bobbin threads for example. It not only maintains the dealer setting 'as is' on one, it also guarantees that you can still sew if you should happen to drop/lose the screw while adjusting the other.
Last edited by ghostrider; 04-05-2015 at 05:14 AM.