I haven't "washed" any pellets, ever.
All of my guns, Co2, pump, springers and rams haven't lost any accuracy with dirty pellets. In fact, all of my guns are still holing zero since the day they were mounted with their scopes, with dirty pellets, years ago.
Some say it does make a difference, personally, whatever, I'm not washing them. My guns hit what I aim at with dirty pellets, and it's just more unnecessary work.
But if you feel you need to wash them, have at it. If you're going to shoot in competition you're going to need every advantage you can get. Can't hurt. Maybe.