In Trek, never.
In other games, it very much depends on the campaign style. Plenty of PC's died when I ran D&D and Paranoia (surprise, surprise), but otherwise, mileage varies.
I like to run cinematic-style - characters die when relevant to the plot, and usually ok'd by the player (I reserve the right to kill off a PC unannounced for plot purposes though), or in unusual circumstances, as a result of extreme idiocy (but never bad luck). I have no reservations about putting near-dead PC's through hell (literally) before their return to active status though!
Back to Trek, I always think of it as the show. Crew death simply does not happen unless it's a major plot point (end of arc episode/turning point for example) or the player wants it (actor leaves for another show).
Jon
"There are worlds out there where the sky is burning, where the sea is asleep and the rivers dream; people made of smoke and cities made of song.
Somewhere there's danger, somewhere there's injustice, and somewhere else the tea is getting cold. Come on, Ace, we've got work to do."
THE DOCTOR, "Survival" (Doctor Who)