I treat all new additions no matter what. If they have mites, they get treated in QT. If they don't have mites, they still get pre-treated in QT.
I think it's always important to pre-treat new comers.
That's my preventative measure to keep my resident animals safe since mites can be vectors to illness and disease.
I don't routinely treat my resident animals if they don't have it.
However, I do know some of the more larger breeders like Brian Gundy, who do treat for mites monthly or every other month as a preventative.
It's no different than putting a monthly flea medication on your dog, imo.
I think if some of the larger breeders (like BHB or others who regularly have animals going in and out of their facility with no QT) utilized regular mite treatments, they wouldn't have such a large mite problem. With that many mites, it's just asking for potential disease spread....