I have never washed my chickens and don't intend to try!
People who take them to shows do wash and blow dry them, and say they seem to enjoy it. And if a bird has a pooey bottom or is otherwise contaminated, she would need a wash. Otherwise, healthy birds stay clean if they can have a good dustbath and preen their feathers, and you can add red mite powder, diatomaceous earth, or louse powder as required to the dustbath and they'll do a more efficient job of getting it right down in their feathers than is possible if you just sprinkle it on the bird.
At this time if year I would think it was not a good idea to wash them unless absolutely necessary, as the change of temperature from the warmth of the hair dryer to the cold conditions back in the run might be harmful to them and upset their acclimatisation to living outdoors.