I kill the named fiends. Oscar Velasco if I have Boone as a sidekick. Legion because I hate them.

Except for OV and legion and gangs, I usually don't kill NPCs with names. And I don't like killing little old ladies, so I won't be siding with House this time out. And I don't kill Kahns except for OV.

All of the NCR higher ups I kill, in almost every playthrough. Nobody I particularly avoid killing though.

I always kill Benny. Really hard to avoid that one.
House dies only because he has to (if I'm against him). I wish i could spare him. It's a shame to see so much potential go to waste.
I usually let Ceaser live. He's worked hard to build up his Legion. I wouldn't mind it if it wasn't threatening Vegas.
I spare Oliver only so that someone can carry the news of Vegas being independent back to the NCR
Graham and Daniel have never died in any of my games. I could never bring myself to kill them. Nor could any of my characters justify it.
I Spare Mobius and the Think Tank. He is awesome and the Think Tank can be useful to me.
Ulysses, despite being broken, and despite his plans, is reasonable in the end, and so he lives.