Always. Unless I can see a high-altitude source, such as the glacier it's coming from.

As you gain elevation, you need less and less filtration IMO. At least in my Rockies. Something that filters out Giardia is enough as far as I know. I have set up siphon rigs that go through gallons overnight with no effort on my part.

Sometimes I'll get an opinion from park staff, off the record (just ask: what do you do?). That's pretty safe for them. If it's on the record, they have to follow what the legal department decided, in an office tower someplace, to prevent lawsuits.

In an emergency, with a source you believe to be reasonably clean, you might choose to drink deep and let medical science deal with the fallout. Or you could boil tea in a steel canteen.

It's a fact that dehydration will knock you down very quickly. But bad diarrhea is no joke.