I started collecting in the late 1980s, so I don't satisfy your field criteria. However, I subscribed to a well-know price guide magazine during that period, as I'm sure most older collectors did as well, and religiously read every article every month.
I clearly remember one issue had an article that explained that an autograph actually decreased the card's value, because it was altering it. After reading that article, I would never have a player sign my best copy. I would purchase another one, usually one that was not in as good a condition as my main copy (because why would I purchase an expensive copy and then have the player's signature immediately reduce its value?), and have the player sign it.
Only if I would have know what I know now.
