Masello's sixth rule is concerned with what he calls "symbolitis," which is the tendency to look for symbols and hidden meanings in works of literature, and then for writers to consciously put those symbols into their own work. It might not be a surprise to hear that he advises against this. He says that writers write a story, not a theme, and that any symbols and themes that emerge from the text will do so later on, and that the writer may not have even been aware of them at the time.
Now on the one hand, I know of what he says. Many is the time that readers have asked me about the themes in one of my stories, and as they tell me what they saw in the text, I realize that I had no idea that the theme had found its way in there. Two of my stories that come to mind immediately are "Kaddish for the Last Survivor" and "Lifeblood;" in both stories, people saw things that I know I never consciously meant to put in there.
But on the flip side, the motivation for many of my stories can usually be boiled down to some sort of theme or premise (to use the word preferred by James N. Frey of How to Write a Damn Good Novel and other fine books on writing). Readers have noted that a common theme in my work is the question of how the future will remember us, and it would be disingenuous for me to claim that I never knew of that theme when I started the story.
On the other hand, Masello's sixth rule also mentions symbols, and that's a slightly different concept. The references in the first paragraph to Ahab's whale and Holden Caulfield's field of rye make it clear that what he's really advising against is writers putting in symbols just to represent something. Frey talks about this as well. He mentions a student manuscript in which every evil character wears shiny shoes, and it's a game for the reader to pick out that when someone in the story is evil, he wears shiny shoes. But out of context, such symbolism is meaningless. On the other hand, a symbol can mean something to a character. Frey gives the example of a poor man who subsists on beef jerky, finally becomes rich, and one day plans to foreclose on a family's house. He goes out to dinner at a fancy restaurant and finds that a piece of beef jerky has been left on his plate, which reminds him of his previous life, which gets him to reconsider. (I may have the details wrong, but that was the general idea.) In this case, the food is a symbol that means something to the character, not just to the reader.
(As an aside, the story I heard about Melville and Moby Dick was that someone once approached him with a whole metaphorical interpretation of his novel, explaining that the whale represented something-or-other and that Ahab's quest was a metaphor for man's search for fulfillment. Melville's supposed reply was, "No, it's a story about a guy chasing after a big fish." Probably apocryphal, but too good not to share.)
My guess is that Masello developed this piece of advice (like many others) from years of teaching writing. Beginning writers often have a tendency to want deep meaning in their stories, and so they fill them chock-full of symbols. When you critique the story and ask them what it's about, they cheerfully explain the symbolic meaning behind the bizarre events -- but if the story doesn't work as a story, the symbolism becomes irrelevant.
My distillation -- if you want to use symbols, make sure they mean something to the characters in the book, not just to the author, and don't go overboard.
Which is exactly what Masello suggests.