404.3. If an effect or rule puts two or more cards into the same graveyard at the same time, the owner of those cards may arrange them
in any order.
404.2. Each graveyard is kept in a single face-up pile. A player can examine the cards in any graveyard at any time but normally can't
change their order. Additional rules applying to sanctioned
tournaments may allow a player to change the order of cards in his or
Basically, in formats where no cards care about graveyard order, players don't have to care about graveyard order either. This rule was created in order to "retire" graveyard-ordering in a graceful way.
DCI-sanctioned tournaments follow section 3.14 of the MTR:
In formats involving only cards from Urza's Saga and later, players
may change the order of their graveyard at any time. A player may not
change the order of an opponent's graveyard.
Thus, you may freely rearrange your own graveyard in Standard, Extended, and Modern. (Word of caution: doing this too often might come off as weird and cheaty. So feel free to arrange your graveyard with all the flashback cards or reanimation targets on top, but don't get in the habit of fidgeting with it the way you might fidget with cards in hand.)