Why did you decide to keep a reading journal in the first place? This might help you come to an answer.

I have taken notes on fiction only a few times when reading for pleasure - mostly in reading sf when some new idea or speculation made me wonder about its further implications and I jotted it down for discussion with other like minded people. That worked fine and I still do it once in a while. I also took notes once or twice when reading fiction as part of a reading group, again with the idea of highlighting discussion points that interested me. But I felt that taking notes detracted from the actual reading and enjoying part of reading - discussion wasn't any different without notes.

It is very necessary when reading certain Greg Egan novels.

Since I have a hard time remembering names (not just of people, I have a huge list of gene names at my desk in the lab) sometimes I need to make a minor-character list for a book.

I have contemplated doing it but never actually done it. The benefit I idly imagine I might get doesn't have anything to do with re-reading the notes later; I hope it might improve the attention I pay and the amount I retain. (I am horrified at the number of books on my shelves that I know I have read but can remember scarcely anything about.) Both of these surely matter less for fiction than for non-fiction.