Butterflyfish are all individuals, but people make the generalization that none are reef safe because none of them are "perfectly" reef safe. People are getting wiser to the fact that it is not a black-and-white issue, as you can tell by how many people play with different angels and moorish idols in their reefs nowadays, but I digress.
Basically, none are 100% reef safe, but some are better than others. It depends on what you value more, the fish or the corals they nip at. That being said, if the coral colony is large and growing steadily, sometimes it doesn't even matter if they take a passing nip occasionally; that's how it works in the wild. This also changes as fish age; many people find rabbitfish are perfectly reef safe until they hit a certain size and start devouring brain corals, and the same can be said for butterflies and angels. The younger ones are usually more adverse to it, but that can change.
My (Personal) List for butterfly reef-safeness is as follows (in order from most reef safe to least):
- Yellow-Longnose (The best in my experience) and Copperbands (Hard to keep)
- The Pyramids (Genus Hemitaurichthys, Gold and Black)
- Certain Heniochus Species (Haven't really played with all of them)
- The "Small-Mouth" Typical Butterflies (as I call them) (Think of the mouths on Raccoons, Aurigas, Pearlscales, Mertensii, Semilarvatus, etc.)
- The "Underbite" Typical Butterflies (as I call them) (Think of Saddlebacks, Xanthocephalus, etc.)
- Obligate Corallivores (Obviously)
These are generalizations; every
individual fish is different. I will say, almost every butterfly I've had in my reefs (save for a few at the top of the list) love to pick at specific groups of corals: Zoanthids are always first, then fleshy brains (Think Acans, Scolys, Lobos, etc.), then the occasional tentacled coral (like the beloved
Euphyllias, Duncans, etc.), then
occasionally they might pick at SPS polyps, but that's rare (again, in my experience).
Hope that sets you in a good direction.