Yeah, use a circular file and grind off the tab on the washer itself. That actualy works better becasue a lot of times, when tightening the locknut down, it spins the washer and the washer gets stuck on teh groove. THen it prevents the locknut from pushing down hard enough to lock everything together and the headset ends up loosening.

Another trick is to use two wrenchs and unscrew the adjustable cup up into the locknut which is being tightened down THis really locks them together and prevents loosing under load.