I think they got better, but for different reasons.
I don't think they were ever looked at by anybody from the advertising world -- they had the Audit Bureau numbers for that, which were theoretically a lot better. I can't see an ad agency skittering around stores looking for Statements when they were paying the auditors alerady.
But what you did have was the Independent Distributors like Curtis improving their speed and accuracy in reporting returns -- the information that the publishers needed to file accurate Statements. I mean, today, a magazine publisher can log into Ingram's system and see exactly how many magazines Barnes & Noble moved last week, what stores ordered how many, etc. Put that against magazine distribution in the 1960s, when the reports were much less sophisticated.
(And, indeed, you had reported-as-unsold magazines disappearing, which is where the Mile High II collection came from. As the story goes, Chuck Rozanski found millions of comics from the late 1960s in a warehouse -- some distributor had used "affadavit returns" to report them as destroyed, intending to sell them some other way. He had to go to Marvel to find out who had title to them, but once he did, it became a good source of high-grade comics from that era.)
Finally, the Postal Service has increasingly asked for additional data in their forms, so I think that's forced the publishers to be more on their toes and find out what they can.
[Originaly posted May 1, 2007]