To summarize the answer I gave to Who can determine what is 'canon' before it got closed - I think canon in some cases is subjective but there are many notable exclusions to that.
In cases where only one author has written books in a world, canon is fairly straight-forward - did it appear in that author's writings? Canon for the Culture novels by Iain M. Banks is fairly clear cut. Same with Alistair Reynolds' books or Modesitt Jr.
Harry Potter is an interesting case as although only Rowling has written in that world, her copious use of Word of God and how it affects canon can be portrayed as a matter of opinion. In general she doesn't contradict her own writings though.
More interesting cases include those I mentioned in my answer, in a spectrum of roughly most straightforward to least:
- Tolkien - the most common definition of his canon is his work, plus Christopher Tolkien's publications. This was Tolkien's intention in setting up the estate, so I'd argue that it would include those, but I can also see how someone can argue that only Tolkien's actual writings apply.
- Adams - And Another Thing..., although endorsed by Adams' wife, wasn't explicitly set up by Adams (it was initiated by the publisher with the estate's approval) so opinion definitely comes into it here.
- Conan - Not particularly familiar as to what the relevant opinions are here, but I can easily see some authors/portrayals being considered canon to the exclusion of others.
- Star Trek - Covered elsewhere, but their concept of canon definitely has degrees.
- Star Wars - This apparently has a crazy number of degrees of "canon-ness".
All through that spectrum opinion comes into it. In this case, it really does come down to the individual fan - I may insist that only the films are canon for Star Wars, because they were what Lucas intended. And that Greedo shot first because of that belief. And that's going to differ from probably every other person in the world. But it's a valid opinion given the definition of "canon".