Hmm, what does this remind me of?
http://www.youtube.com/watch?v=IMk5sMHj58IUnless it is a BYE week for your fantasy team, you are playing a game - put in your best lineup. If you're not doing that, you have no ethics and are not welcome in my league. Why is this unethical? Allow me to elaborate...
You play the Jerks in Week 2. He plays his best lineup and beats you. He goes on to secure the #1 seed by week 12, and plays the Spitwads in Week 13. (The playoffs begin Week 14.) Now, the Spitwads have had a ridiculously easy schedule, somehow playing every other team on their off week. In fact, the Spitwads have had the fewest points scored against them in your league.
You beat the Spitwads handily earlier in the season, thus own the tiebreaker. You currently are one game back from him to earn the final playoff spot. If you win and he loses, you're in the playoffs. You log in on Sunday to see the potential matchups only to find that the Jerks, who played Brady, Addai, TO, and New England's Defense against you, are now playing the injured Grossman, Gado, Harrison, and the Jets D against the Spitwads.
You win your game, 118 - 103. The Spitwads beat the Jerks, 61 - 44.
Now do you think it is ethical for the Jerks to tank the game? Is that a league you want to play in?
It is slightly different in the NFL when you can actually prevent getting your stars injured unnecessarily. In fantasy, you can't, so you should just play them. Besides, it isn't like an NFL coach is telling his backup players, if that's who he decides to play, to go out and lose the game so that the Vikings don't make the playoffs. They are still trying to win, just without jeapordizing their best players needlessly.
A fantasy team does not jeapordize a player's health by playing him, so he should be expected to start his best lineup every time, regardless of the circumstances. You wouldn't allow a 1-10 team to tank a game and screw up the playoffs...why allow a 10-1 team to do the same?
The Commish always puts precedent on what is best for the league...not one individual team. The fact so many people think that tanking a game is OK absolutely makes me sick.