I've always found that in places where the sunlight doesn't seem obviously weird---oblique winter light, heavy filtering from Beijing smog, stuff like that---sunny 16 is dead on. Even at the low-temperate latitude of San Diego, I usually get sunny-11 conditions in winter. Sunny 22 is an extreme case, light-sand beach/desert conditions in full screaming sun, but I have seen it happen.
With negative film, I shoot on sunny-X principles all the time. I don't have the courage to do it with slide film for the most part.