I've never held back from pressing the shutter. Fortunately I've never been put in the sort of difficult situation that I've needed to have to make a really difficult decision.
I have no idea where the line is - I just hope that, should I ever stumble across it, I'd notice it.
This sort of thing often leads to discussions about Kevin Carters "Starving Child Stalked by Vulture"
Carter saw it as his duty to record the suffering and, apparently, only had the chance to either shoot or help. His decision tormented him so much that he shot himself about 3 months after taking the shot.