I've also had my fair share of older Fender. I agree with all said here. The quality problems in the '70s was more of imperfections (misaligned bridges, imprecise neck pockets) but in general I'd say that the quality of the wood and the hardware were very good. Whether you like the sound of heavy ash or not it's a matter of taste, but I also think that wood gets better with age (even under thick poly) and "usually" when I hear or play an older Fender, it sounds better than a current one.
My '82 Jazz (which was built basically from late '70s parts) has such a magical sound, full, complex, etc, etc, that no Sadowsky or Lull or Lakland can surpass. I once had the chance to play an all-original lefty '65 P-Bass - and equally, it had a sound that was just incomparable. I don't believe in magic, but I do believe that wood sounds better with the passing if time. That's the only explanation I can find.
So, when buying an older Fender I think that the most critical part is that the neck is in good shape. Frets can be changed or dressed, electronics can be changed, neck pockets are not an issue, but a warped/bent neck is a problem that will render the bass unplayable. That's not to say that there aren't '70s "dogs" out there made from bad wood - I'm sure there are.
Another thing that stands out for me on '70s Fender are the necks. I love those wide frets and maple necks - they sound right, they feel right. The necks in my '82 Jazz (has a soft V profile) and "A" neck in my '73 P are just to die for. I know that most '70s P-Bass have really thick necks, but I still love the feel of them.
And to answer the original question, I don't really think there is a definite worst year/period in the '70s for Fenders. I think that late-'70s tend to be the heaviest, but that's not 100%. My '82 Jazz (again, made mostly from leftover '70s parts) weighed 9 lbs before I put Hipshot tuners), and the same with my '73 P. With the Hipshot they are now around 8.5 lb. . .