As per my comments in the Sasha Grey thread, I believe she's probably smart enough to save her back-up (dirt)chute for when she really needs it...she has a strong enough fan base that her 1st anal would be worth a little something extra to a producer who knows the value of having something 'must see' in their flicks and is prepared to pay extra for it too, like a Jules Jordan.
Then, after anal, she still has the DP card left to play too...