What are (plain) washers for?
They spread the load of the bolt's head over a wider area. Useful if you are bolting a piece of wood or soft or thin metal.
They prevent scratching of the paint of the item being bolted.
They allow spacing the bolt's head higher off the item being bolted. This may help with access to the bolt head.
I suspect they don't specify because it doesn't matter. If you really want to know for sure, Dillon's customer service is said to be the best.
Personally, I would use the washer. Cost is a few cents. Because there is (appears to be) a raised area at the bolt hole, cleaning under the edge of the washer might be a problem, but if you use a washer the same size as that raised area's diameter, that should be cured.
Lost Sheep
Congratulations on a fine press.
Lost Sheep