I am on my third D600. I returned the first two for back focusing issues, not dust on the sensor. This one focuses perfectly with no fine tuning. It had one dust spot on it out of the box, and now after just 200 actuations it has about half a dozen, mostly but not all in the upper left. I do not want to return it, but if I wet clean it and it continues to show increased dust spots without changing lenses, I may do so. But then what? I have yet to see anything definitive about recent serial numbers from the factory being OK.
I have never had a Nikon camera on which using the "clean image sensor" button did a thing even when doing it multiple times in a row. Does that ever work for anybody?