I voted always. Would I never ever let a PC live who the dice and situation killed? There might be some circumstances, but they would be so rare as to be about zero chance.
Like creativehum and others have mentioned, having additional characters (whether NPCs or second PCs or henchmen) around makes it easy for a player to just pick up a new character, or they can wait until it's logical to bring a new PC in (maybe running an NPC in the interim).
There have been times I went out of my way to save PCs, but with my preference for systems that are pretty easy to generate a new character, my thoughts have changed. Now sometimes I play Burning Wheel and in that game it's pretty hard to get your character killed, but that's a game with specific mechanics, it's not the GM saying, "Oh, Bob is going to be so crushed to lose his PC, so I'll do something to save him."
Of course it's also worth noting that Book 1 Traveller combat (especially with just Book 1 weapons) is actually hard to actually get PCs killed with one shot. Sure, if they go down, they might not be rescued, but there's still a chance of the rest of the PCs doing something (or maybe the antagonist doesn't actually want them dead).
For OD&D, I have a negative hit point rule that gives at least a chance of surviving, most PCs who have gone below 0 have died. I've lost some players as a result of these deaths. Oh well, go find a game that is more to your liking, there's plenty of people wanting to play my game.