Nellisir
Hero
You tell me. I have fudged rolls and kept PCs alive, though not from any grand sense of "story", but rather because it would have been bad for morale (of the players. Few things suck more than dying a pointless death after a night of crappy rolls and stale pizza.) But I like to run "quest" campaigns, where the PCs are out to accomplish "something" in particular. I don't think I railroad too much, but it's usually clear where the adventure is and where it isn't. I have a general sense of what should happen, both in the adventure and in the campaign, but the goal is never dependant on the survival of any particular PC or group thereof.