My guidelines...
If there a death, then let it have meaning...it shall not be CHEAP!
Letting the players know in advance, that something might be bad around the turn, make them work harder to overcome the challenge.
Let the Dice fall where they may...is a cop out. The DM/Storyteller is the final law on all things in that verse. So, anything can be as wanted, as seen fit.
I don't believe in total TPKs...unless,as stated in this thread, the players put themselves into it. On that note, they have to be a bunch stupid SOBs to go there.
When you get hit with a bad blow, you suffer the problems that come with it. Lost armor piece, damage body part, strength lessen, nothing instant kill, again nothing cheap.
And lastily, having a decent size and very smart group of players, who understand the ramifications, when it comes to combat.
That goes a long way.