When should you not place the robber to do the most damage in Settlers of Catan?
The robber is a political action.

If it's early days in the game, you may well want to shrug your shoulders and say "I don't care, I'm just going to put this where it will cause the most damage to the most players who aren't me. Nothing personal."

If it's later on, you might want to attack a player who's been giving you hassle, and make it clear that that's what you're doing.  Or you might want to avoid doing anything to hurt a player who's behind - they may be grateful to be "given a break", and return the favour later.

Naturally, if another player is about to do rather too well, putting the robber on him may be everyone else's last chance not to lose the game.  Or if you're about to grab the last few points necessary to win, then a well-judged resource-steal may be a timely action.

But in general I don't think you need to micro-analyse your robber plays.  Use the robber to make friends and punish enemies, and maintain some approximation of a status quo (until you're ready to break free of that).  Sometimes it can even be advantageous to place the robber where it will do the least harm.  Everybody likes a nice person, right?  Where I come from, it's not nice guys who finish last, but overly competitive ones :)