When in any zone other than the battlefield (and after having been transformed), double-faced cards have only the characteristics of their front face (‘Day’ side). While there is not a penalty associated with accidentally putting a double-faced card into another zone with the wrong face showing, doing so has the potential to create mistakes.

For example, the Dark Ascension card Ghoultree cares about the number of creature cards you have in your graveyard, because the cost to cast it is lowered for each one of them. There is a double-faced card that is a creature on the front but an enchantment on the back (Soul Seizer/Ghastly Haunting), and one other card this an equipment on the front but a creature on the back (Elbrus, the Binding Blade/Withengar Unbound). When one of these cards goes to the graveyard, it is put there face-up. It does not matter if it leaves play with the back side facing up. When going through your graveyard, it is best to keep these double-faced cards face up. It is easy to miss the Ghastly Haunting as a creature card if its back face is showing in the graveyard, just at it would be easy to accidentally count Withengard Unbound as a creature card instead of an equipment card in your graveyard if its back face is showing.

This can also apply to effects that count the creature subtypes of cards, as most double-faced cards have different creature subtypes on their two sides (e.g., most Werewolf creatures are Human on one side, but not the other).