from how i understand it the noms are USUALLY decided by comp scores... if keys arent in the equation at all it's usually the lowest comp scores in danger for nominations
but somehow thru the algo, with a key you can still score lowest and be safe
i dont think anybody exactly knows how keys work but a lot of people tend to believe that if you SHOULD have been nominated but a key saved you, it "uses up" that key and you're in danger again unless you have more than one (im not sure how true this is though - might be more complicated)
so if you truly are coming last in every comp you prob would need a key for most days the game lasts LOL
and then if you make it to final day final comp score matters a lot so if you flop it your chances are not so good
so long story short - i think keys are important to GET you to final day, but you probably need the checks and comp scores to back you up to actually win
if you consistently win comps and are fist in checks