It doesn't matter. Federer will definitely not tank.
We'll see. It could be wise to, though, Nole or Murray will probably be waiting for him less than 24 hours later. The group B is at a huge disadvantage in this respect, especially Federer who due to age and mileage can't recover as fast between matches as he once did.
Well, beating Del Potro will probably mean eliminating him, so he won't have to worry about seeing him again in the final. He wouldn't mind seeing Ferrer a second time.
There's also the possibility of Tipsarevic beating Ferrer, in which case a loss to DelPo will give away the group win.
But is getting rid of a potential threat in Delpo more important than being fresh? I mean, if he and Delpo get involved in a long match like Olympics or Basel, odds are Federer will not even be in the final.
As for the group win, does it really matter? I might be wrong here, but don't think has any preference between facing Murray or Nole in semis (it could be relevant if Jo/Tomas snatched second place), he'd probably even prefer second place to play the first semi and have more rest time for the final if he wins.
Obviously this is just looking at a situation from a practical point of view. Group B is kind of screwed up by the scheduling, I'm sure Federer will at least have this in mind.