It's hard to make a general rule of thumb.
A problem that will sometimes happen on TrekBBS is where a reader will ask about a particular older title, or mention that they are about to read it for the first time, and someone will chime in with a massive spoiler before that newbie had a chance to read it. Then there will be outcry and sympathy, and the public defense is usually "But it's been out for decades!"
Similarly, some novels have crucial spoilers that don't necessarily sound like spoilers, and it can be dangerous for people who haven't actually read
those books to discuss them without innocently revealing things they've "heard".