Totally agree about the Permethrin spray! It's so easy to use compared to dusting! (Although, spray isn't a good thing to use in the middle of winter if you live somewhere cold, so we still have to dust half the year. lol)
Just check them once a month for bugs and don't treat them unless you actually see evidence of the bugs. I think checking them in the dark with a strong flashlight is the easiest way to see mites. They may not ever get them, but it's good to stay on top of it if they do.