Basically, and there are a couple of people who will strongly disagree, but it's like law: precedent is authoratitive but the latest law from a competent legislative body overrides conflicting older ones.
So sources from the 70s are great if there's nothing since then to contradict them: one of the amazing things about Traveller is how much detail there is for the Official Traveller Universe. But if the 1977 version says you need x% of the ship for a given component, and Mongoose (who own the rights now) say it is y%, then you can say it is x, y or z but canon is y%.
You get people saying "argh why don't the writers read the old versions?!?" but in many cases it's safe to assume that they do (in the case of MJD, who gets that a lot, he probably wrote the previous version). Geir knows his shit. Paltrysum knows the fiction of huge areas of the game better than anyone after so long. They've just decided to revise it, which is fine by some but anathema to others.
Edit: there are versions of Traveller that are later than others, which conflict with them, and which everyone sane just agrees to ignore (heplar drives lawl).