The KOMs look to have a pretty meaty spoke bed, so maybe washers are not necessary.OK thanks.
So would you still use washers under DT nipples on other rim brands?....do all wheel builds use washers all the time?
(I edited my first post when I realised how it read....Washers are for nipples, not spokes )
Some rims have eylets in the spoke holes to negate the need for washers. Washers help distribute the load over a greater surface area so should in theory make for wheels that stay tensioned longer ie. the rim won't deform around the spoke holes as much and reduce spoke tension. Then there are things like the DT Swiss Squorx nipple with the PHR washers are an 'engineered' system, you should use them together. Sapim has something similar too.
The only disadvantage of washers would be the additional weight which is bugger all. A tiny bit of grease on the nipple will allow the washer stick to it and also reduce nipple/washer friction as you tension up the wheel.