I'd have to agree that reps are situational. As Buzz Lightyear points out, if you're attacked and you win, it seems morally acceptable to ask for reparations from the aggressors.
However, as Methrage points out, its a tougher decision if you declare on an alliance, defeat them, and then demand reps. To me it would make more sense to demand the reps first and then if they go unpaid, declare war. (Then you would have valid reason to ask for reparations).
Demanding reps from someone simply for honoring treaties is, well, even more situational. It depends on what your goal is; if you really don't like the alliance you were fighting, demanding reps from their allies would be a good way to try and get others to cancel treaties and isolate your target alliance. However, if you have no personal dispute with the alliance you were fighting, I see no reason to ask for reps from their allies.