I wouldn't put a washer there unless the axle slipped without one. If I did put a washer there I would consider using a toothed washer, either the external or internal type. Of course most of us minibike builders aren't racing our bikes so the slight weight difference probably doesn't matter. Having said that, I always like to bear in mind that Colin Chapman, designer and founder of Lotus cars, and perhaps the king of building light, would raise all kinds of hell if he caught a mechanic installing a washer where it wasn't, in his view, needed.