I don't think there are any set in stone rules regarding this - with Fenders, I think you have to judge each instrument on it's own merits, as I've seen killer basses and real dogs from each and every era... This goes for the entire Fender line, including the MIAs, as well - granted, overall quality will usually be consistantly better with the MIAs, but at an increased cost... I had an early 90s MIM P Bass for years that was great - but played a few lemons from that time period, too... I had a late '90s MIM Active Jazz Deluxe that was great - and currently have an '06 MIM active Deluxe P Bass Special that is as good as any Fender you'll ever pick up...
I went on a buying spree around a year and a half ago - and bought a total of 13 basses... or, more accurately, I ended up with 7 of them... I returned 6 basses, 5 of which were Fenders - some things were Fender's fault, some were GC's fault... This was while buying from GC's used gear page, and ordering mostly out of state, and going by the idiot on the other end of the line...
Anyways, the point is, Fender's *can* be hit or miss - but when you get a good one, they are real work horses...
- georgestrings