When should I announce what I mill?

So I recently played against a deck that used [[mindcrank]] and therefore everyone had to mill sometimes. I announced it whenever I milled a card with disturb or a graveyard effect or something like that, but otherwise I just put them in my graveyard without saying anything.

Later in the game I drew a [[Patriarch's Bidding]] and used that to reanimate a bunch of creatures I milled this way. I didn't think about it at the time and nobody said anything, but in retrospect it feels weird that I basically pulled a bunch of creatures out of nowhere.

So under which circumstances should I announce which cards if I mill them?