If you're really roleplaying (vs. powergaming), then you would let your alignment and characer's personality drive the decision irrespective of the reputation score which that decision will yield. If you're powergaming, then your reputation is a tool which determines prices, the mood of the people you deal with, the number of "wanted" posters with your name on them, etc.
By the way, if you can figure out only one way to deal with a situation, that way is neither roleplaying nor powergaming - it's just the only way to move on in the game.
