Wait, I totally misunderstand the rules of Dominion then!
My group has always played that during cleanup, cards from play get discarded first, and then the remainder of your hand in any order, so you always have to reveal at least one unplayed card.
EDIT: Just checked the rulebook, and while it's ambiguous, and I can see why we read it how we did. The salient sections are
"The player places any cards that are in his play area (Action cards that have been played in the Action phase as well as Treasure cards that have been played in the Buy phase) and any cards remaining in his hand onto his Discard pile. Although the player need not show the cards remaining in his hand to his opponents, since he places the cards in the Discard pile face-up, his opponents will always be able to see the top-most card of his Discard pile."
and
"The player places all cards in his play area onto his Discard pile. This will include all Action cards and Treasure cards he played during this turn. He also places all cards left in his hand onto his Discard pile."
Especially in the second description, where the two actions are described in separate sentences and it says "He also" to talk about discarding your hand, it seems really reasonable to read it as specifying the order. But clearly it doesn't actually say that, and I guess that isn't what was intended. Which I think is a little said, because it kind of needlessly removes a source of information (but only if you are crafty enough to pick up your cards in play and put them in front of your cards in hand, which is kind of unnatural).