Bobbin washers are for back lash and don't really do anything for your tension.
That all has to do with your bobbin tension spring and your top tension. If you tend to get loops on the underside of your quilting and you have a metal bobbin casing that does not have a bobbin spring - then a bobbin washer is a good alternative!
What it does is when you stop or start or move very quickly sometimes your bobbin will spin after you've stopped and create little loops. The bobbin washers in the right bobbin case will prevent that. Good luck!