I have varnished very few of my paintings. Its value is to protect the painting, as it can be removed, taking any accumulated dirt with it. Free of kerosene lamps, candle and coal soot, our homes are much cleaner now, and protective coatings are less necessary. Moreover, varnish is extremely toxic and is normally sprayed on. A process like that necessitates outdoor application at the right temperature and humidity, so it's the rare day that is suitable. On the other hand, creating an reason to celebrate is never a bad idea.
Still cogitating this one. Please weigh in, artist friends.