I personally never treat anything that doesn't exist yet. I don't think cooties actually exist anywhere?! However, lice and mites and some other little buggers do exist so...
Having said that I am also proactive and use DE (food grade only) and sprinkle it on the floor of the coop and in the nest boxes and occasionally over their feed to help prevent said cooties from possibly appearing. Some people sprinkle it in their livestock - I'm thinking of doing that this summer myself.
So, I think I just contradicted myself... I do treat for potential cooties but only by using DE which is harmless and helpful and not chemicals.