Do I need to reveal a face-down card if it is put on top of my library?
Yes, you must reveal all face-down cards as they move zones or when you leave the game.


  707.9. If a face-down permanent moves from the battlefield to any other zone, its owner must reveal it to all players as he or she moves it. If a face-down spell moves from the stack to any zone other than the battlefield, its owner must reveal it to all players as he or she moves it. If a player leaves the game, all face-down permanents and spells owned by that player must be revealed to all players. At the end of each game, all face-down permanents and spells must be revealed to all players.


The other players must be able to check that you were able to cast those cards face-down. Making an exception for manifested cards (which are not required to have the morph ability or any other property) would needlessly complicate the rule, and if you have both manifested and morphed creatures on the battlefield, it might be obscured which was which, again making room for error or even cheating.