I can't think of any butterflies that are sure-fire reef safe fish, but alot of folks have had good luck with Copper Banded Butterflies. The Heniochus Bannerfish is closely related to the butterfly family and is usually considered reef safe.
I WOULD HAVE TO SAY TO MONITOR THEM, BECAUSE MY HENIOCHUS EAT ALL MY FEATHER DUSTERS AND PICKED ON A MUSHROOM THAT WAS ON A PIECE OF LR I JUST ADDED THIS WEEKEND