Text (SH+errata): Whenever a creature is put into your graveyard from the battlefield, put that card on top of your library. [Oracle 2009/07/08]
This is not an optional ability. [Barclay 1998/02/27]
If multiple creatures die at once, you resolve the triggers in the order you choose, so the creatures end up on your library in the order you choose. [Barclay 1998/02/27]
