My chickens had mites last year. I used sevin because I had it on hand. I think the better option would have been DE (diatomaceous earth). But it worked for mine, and my chickens faired just fine. I wore disposable gloves & applied on each chicken.
You don't need to use it unless you see bugs on them. It's a waste otherwise. DE was a complete waste of my money as well. It will not treat an infestation. Some people use it as a preventative. You'll have to try it to see if it works for you.
I agree with Welasharon--don't treat if you don't have a problem. Save it for when you find mites or lice, and inspect your chickens often for weight, checking for ice and mites, inspecting crops and vents.