Is reality necessarily subjective for Winston? Can there ever be an objective account of history and its events in 1984, or is reality dependent upon the observer?

Reality is absolutely subjective, based on the way in which the news keeps changing depending on what is happening.  From the point of view of Big Brother, it is objective because it is "what the Party says it is."  However, by definition, if it keeps changing it is not truly objective, because that would mean that once it is said, it is not changed.