A washer may not provide a decent purchase for the nut, and on some bridges may ding the back of the bridge. Also, it doesn't provide stability at the back of the bolt. This isn't a problem on the fork, but could be on the rear where there isn't much purchase to begin with.
In your shoes, I'd buy 2 recess nuts, drill out the threads to 6mm, and use them as support bushings to for the bolt, and double as a washer.
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