Whenever I balance a prop I sand the back of it down.

Also if you use master-ariscrew nylon/glass props I'm told you should sand off the letters "master airscrew" on each side. The letters can come off if fuel hits them, and cause an imbalance, especially if you are flying. An experienced instructor at my airfield told me this, so please don't spam me if I'm wrong. Hope I helped.

-brian