In the last game I played, I got to the end of sokoban and cleared out the zoo only to find that the closet that should have had my reward was empty. I searched all over the level and couldn't find it (I had stoned most of the monsters with a cockatrice corpse, including the one who had been carrying the BoH). Since @'s generating in the treasure zoo are not at all uncommon, this seems like it could potentially be a pretty big issue.

My recommendation: apply the scare monster immunity only to uniques and members of the "dungeon staff" – shopkeepers, guards, priests, all @ that will incur murder penalties if you kill them.