440, 441, whatever it takes. Seriously though? When tuning a guitar? Or any "stringed" or "performance" instrument, it is very difficult to get it exact? And even if you do, as you play an acoustic guitar, it is warmed by your body, and the strings are affected. So unless you tune each string, using a strobe, after every few measures, it will float ever so slightly... But it doesn't matter because you can't hear it, and neither can anyone listening to the performance. What is key is that it sounds good in the end, not the number of vibrations in a given key.