No matter what you do this weekend you still have Pettitte, Wang, and Clemens pitching. After this weekend you can see where you are and then decide whether you want to go with a five or six man rotation. Obviously if the race tightens, you want Pettitte and Wang going every five days.
By the way, I think you are misunderstanding what Jacko is saying. He's saying, in my opinion, that the Yanks should do everything they can to win the division. He's not saying anything about simply getting there.
In my opinion, preparing yourself for the playoffs is much more important than busting your ass for seeding. I'm not saying the wild card race is over by any means, but if it begins to look more comfortable, I think a six man rotation would better prepare the Yanks for the playoffs, no matter how close you are to the Red Sox.