Well, what if I don't see the super-psychic guards when I'm done? Will they still come after me? I'm trying to play my character as straight an arrow as possible...but there's nothing that says Imoen can't do anything...:-p
You will first see a message that somebody has noticed you. That is the cue to take an invisibility potion, or leave the area immediately. So long as the guards don't get the chance to initiate the "I SERVE THE FLAMING FIST" dialogue, they won't turn hostile. And even if they do turn hostile, you can leave the area (through a transition door, go upstairs/downstairs etc.) and they won't follow you, neither will the entire Flaming Fist be on your tail. There will be a consequence of them turning hostile, though: you only get the option to bribe them, or fight them, and you lose reputation (-1 point) for that -- kill them, and it plummets (-5 points).
Edit: Oh, and it doesn't matter which member of your party
commits the deed, or gets caught: you're all in it together.