I think if headshots were really the big, bad bogeymen that some people in this thread make them out to be then the last Space Marine scout would have probably died in the 31st Millenium and all of the other Chapters would have long since gone extinct through attrition. From a mechanical perspective it is incredibly easy to construct an antagonist that can make headshots with incredible predictableness, but then I suppose you need to really sit down and consider the implications of what you are doing to your own game and the cognitive dissonance you are going to create viz a viz what happens in your versus what happens in the source material. And, of course, you need to think even harder about the gaming environment this sort of behaviour will engender - some players will retaliate by doing their utmost to be unsurprisable initiative winners who are likewise optimized for headshots.
Frankly, I am never going to play a Space Wolf because of how incredibly unattractive they are. Wolf Senses is the biggest point that has me resolved never to play one, because of the tremendous advantage to wearing a helmet and the tremendous lack of incentive to not wearing one. I am in all honesty quite glad I do not and will never have occassion to play with many of you because the heavy simulationist impulses towards aggressive punishment for in-character but disadvantageous choices is utterly appalling.