Here is some counter-intuitive pragmatism. The best time to tune your piano is when it sounds its best (not when you're in pain because it sounds terrible). The less the wire has to be moved and the more it's being refined in the same place, the more solid the end result will be. So, with or without a Dampp-chaser, it's arguable that you'll have more musical days per year if you tune it once a year in the same moderate humidity each time, than if you have it tuned twice a year changing it back and forth. But in New England, the weather could be almost anything on the day of the tuning, even if it is the "temperate" season.

So. The best approach is to support that temperate strategy with Dampp-chaser insurance. The closer your soundboard's weather-of-the-day to, say, 42% relative humidity when the tuner appears, the better the job that will be left behind. And that's just where the Dampp-chaser tries to keep things.