I'd disagree with the "Must" part. I say treat when you have a problem. Very often, in my experience, newly established colonies don't need treatment the first year. Have you done a mite count? Do you know what the threshold is for treatment in your area? Mite counts generally increase in late summer, and I usually treat when I see a 24-hour drop between 40-50.
You DO want to be sure that heading into winter you have a strong, healthy colony, so if mites are a problem, decide HOW you're gonna treat. The "How" will often answer the question of "When". For example, since I treat with powdered sugar, I tend to treat late summer. If I were using something like Apiguard, I'd probably wait until early-mid fall.