C.C. and Sheets should not pitch back-to-back
The top five teams in the baseball for innings pitched from the rotation are the Angels, Jays, White Sox, Red Sox, and A's. The top five teams in bullpen ERA are the Phils, Jays, Dodgers, White Sox, and A's. That's three teams out of five that are on both lists. You see similar correlation at the bottom of both lists. I think it's intuitive and obvious that a rested bullpen pitches better than a heavily taxed one. But I also think that you can have too much rest (See ESK's discussion of why platoons work below).
By my calculations, Parra averages 5.67 innings per start, Sheets averages 6.74, Suppan the inning-eater averages 5.7 per start, Bush averages 6.28, and Sabathia averages 7.05. Sheets also has 3 complete games this year and Sabathia has 6. None of the other starters has a complete game.
While watching C.C. throw his sixth complete game last night, I was thinking about Sheet taking the mound tonight. It's possible that he could go 9 innings. It's likely that he'd go farther than anyone else in the rotation will go in their last or next outing. It's also more likely that we'll be winning when he does come out of the game. Does it make sense to let Sabathia save Torres et. al. a night of work so they'll be rested for... another night off? Why not stick Parra or Suppan between them? That way, you'll have more non-mop-up-duty relievers with a day of rest ready to step in for the middle guy. From the other angle, if you have Bush (or McClung), Parra, Suppan all in a row, you probably won't have Torres (and whoever we decide is the best 8th inning guy) ready for the third game if we happen to be ahead.
I think the rotation should go Sabathia, Bush, Sheets, Parra, Suppan. At least until the playoffs.