it's a bit of a grey area,. some people claim they can put their graupners on without the need for washers..
yet others insist on it, some vendors sell washers with the graupners,.and there's even a video online where someone goes thru the trouble of using a dremel to scruff up steel washers. (8mm washers) - with nearly 1000 hits!#.
I have yet to put my graupners on, but i bought the washers and found them to be a bit loose fitting.. with all the talk about balancing motors i wasn't too thrilled tho.
so I found some FIBER washers, that were 5/16" inner diameter/ 3/4" outer diameter, and they fit perfectly, and are a bit scratchy as well., so I'll go with those. (along with a few drops of loctite of course)