I only bath mine if they are really dirty and are stuck with mud or/and poop in places that won't easily come off by itself.
Some breeds get dirtier then others.
For example, my polishes crest can get dirty very easy and he can't clean it himself.
Japanese also get kinda dirty because there legs/feet are so close to the ground they sometimes drag their wings in muck (but i imagine this is something they could preen off themselves if needed).
For hygiene in general, chickens take dust baths which help rid and prevent lice and mites.
So in general, chickens don't normally need baths.
But special circumstances can occur when they do.