Sean is of course, right. I was referring to if you had any imperfections on them, sometimes, if you use a split-lock washer with a flat washer, you get a burr on the flat washer from the split-lock washer doing its job. That's when the sanding and heat/quench helps to make a stable, uniform hardened surface on the washer.
I used to heat/quench the head gasket on my two-stroke Yamaha 360 offroad bike, probably not necessary if it wasn't warped. But, who doesn't like using their tools?