I too have come to the Zone IV shadow placement rather than the "classic" Zone III placement; this is for shadows in which I expect plenty of readily-visible detail, not the darkest thing I can find in the scene. This isn't after a specific incident, it's after an embarrassing number of years of feeling like the shadows may be printing a bit too dark too often.
I usually use the film's normal EI or close to it. Many people rate the film at about half its usual rating and place the shadow exposure on Zone III, which is pretty much the same thing.
I believe I read somewhere or other that Bruce Barnbaum had analysed an assortment of Ansel's photos and concluded that Ansel had frequently used the Zone IV placement even though he said or implied that the placement should be Zone III.