Does OL need to tell heros which monsters he will put on the map before he does it?

[notaprofi 0]

For example in the Masked Ball second encounter, you don't place monsters until the door is open. But do you need to show players what kind of monsters they will face in the room at the very start of the encounter? Or you can pick whatever you like the very moment the door is opened. 

[Sadgit 574]

Open groups are chosen during setup and the OL must reveal the monster groups at that time. When a figure enters a space on the Goblin Cave, the OL may choose one of this open groups to be placed.