I disagree. I think a DM should design challenges that allow for the player to make choices that increase or decrease the difficulty based on their choices. That is part of what makes a challenge satisfying because it gives those choices meaning. At the same time, difficulty is also part of what makes a challenge satisfying. If the players have found some scheme to reduce difficulty to unsatisfying levels, however, they have a hand in their own dissatisfaction. The DM can then set up encounters to counter these tactics specifically, but this often results in something of an arms race. A better solution, in my view, is for the players to be cognizant of the goals of play and to make choices accordingly. Sometimes that means making the most optimal choice. Sometimes it does not.
Again, I think self-policing as it relates to making suboptimal choices flies in the face of human nature, and that's assuming all the players and the DM can come to a consensus in defining what is reasonable and what is broken. Furthermore, the DM is walking a thin line if he is expecting or encouraging players to consistently moderate the difficulty of any given challenge. Complacency quickly sets in if the players believe everything will work out for the good of the story. There's no tension if the threat of failure is largely removed, and for a lot of D&D players, no tension equals no fun.