I think it depends on the opponent and on how strong your bench is. If your opponent has 3 beatable starting pitchers for a series, maybe you rest one starter each game and try to win all three. If your opponent has one stud starter out of the three, maybe you sacrifice the game the stud is starting by resting three starters that game and playing the other two full strength.
With this Sox team in particular, Flowers starts are going to come vs. LHP, regardless of which game in the series it is, and other than Dunn, Konerko, and Alexei, I don't think any of the other starters are so good that you lose much by subbing in Fukudome, Escobar, or Lillibridge.