Never a great fan of the upholstery-type washers often used on wood rim, we despair over the low quality and frequent failures. A better solution are these 18-8 stainless washers that are black oxide plated.
Why stainless compared to brass? The friction coefficient between like metals is often very high. Stainless against stainless, brass against brass, aluminum against aluminum. These surfaces are more prone to galling (micro welding) under high pressure. Brass (nipples) against stainless (washers) has very agreeable friction.
The important criterion is washer size. It needs to be just under 10mm OD for most (Ghisallo) drilling and just over 3.8mm ID for nipples. These washers are perfect.
They also work well for metal rims.