In Back to the Future, it's generally accepted that Time is changeable; it's the whole theme of the movie. Because Marty taught George not to be a selfish recluse, he became a sucsessful father and author. History changed. Remember the photo, the news papers used through out part two? They change when the past changes.
By that logic, Doc is technially dead until Marty changes the past (The fact that the Twin Pines Mall reads "Lone Pine" before Marty goes back in time is an inconsistency--but cinematically it is better as a subtle thing.)