Originally Posted by
Piff
Honestly, it's not needed and I find it's better not to use a keyed washer, just a regular washer. The keyed washer may make it easier to torque down the locknut, but it's not worth the chance of buggering the threads on the fork. Just use a regular washer and use two wrenches to get the cup and the locknut tight against each other.
I've passed on the washers both for lack of one on hand and to minimize stack but I don't like to. That said, I always use two wrenches and don't move the bottom as I tighten the locknut to protect both the steerer and the nib.