In stealth games, I like to keep hidden and undetected, and won't compromise on that. I will take the time to haul bodies into vents and enclosed areas even if there's virtually no risk of them being discovered in the open. But I do also kill every guard if I'm not going for an achievement or mission objective or something. Half the time it's functionally the same as the non-lethal option, and the other half of the time it's better because they're not going to wake up later. Although I would consider non-lethal more if you could lock the guards in cupboards tied and gagged instead of just leaving them somewhere to inevitably wake up and have all their faculties, something that has always annoyed me about games where the guards wake from tranquilisers.