I'm definitely in the "for almost everything" camp. It's less ambiguous especially when you consider the DD/MM vs MM/DD nonsense between US dates vs elsewhere. Pretty much the only time I don't use ISO-8601 is when I'm using non-numeric month names like when saying a date out loud.
In Canada we use MM/DD and DD/MM so you never quite know which it is! There's an expense spreadsheet I fill out for work that uses one format in one place and the other format in another....