Don’t follow any rule off a cliff.
I recently read an interview with Rebecca Skloot, author of The Immortal Life of Henrietta Lacks. She says, in response to making herself a character in this nonfiction work:
I refused to be in this book. I think a lot of potentially great stories out there have been damaged and in some cases ruined by a writer not being able to step out of the story and let the story happen. When I teach, I always harp on my students, “Stop inserting yourself in other people’s stories.”
Rebecca Skloot’s best selling book, praised by the New York Times, Los Angeles Times, The Wall Street Journal and many other publications, would not exist if the author had followed her own advice.