In Storyteller, Kate Wilhelm discusses a story written by a student at the Clarion workshop in which all the events of the tale take place behind a wall that is too high to see over. Wilhelm asked the student what was behind the wall and she said that it had never come up in the story and she didn't know. Wilhelm says that: "The writer has to know what lies beyond the wall and has to give the readers enough direct instructions or else imply enough through the characters' behavior for the readers to be able to piece it together."
I agree absolutely with the first part of this. The writer should know what's behind the wall because it will affect the actions of the characters. It's like having that metaphorical elephant in the bedroom that no one talks about. I also have to wonder, how could the writer not know? How could you write such a story without evoking your own curiousity about what lies beyond the wall. How could you not have thought about it? Half the fun of writing is world building.
The second part of Wilhelm's comment, however, about giving the readers enough clues to piece it together, is less clear to me. If I was reading the story I would want to know, of course. And I'd be frustrated if the writer gave me no hints and I could never even develop a theory of "beyond-the-wallness." Yet, I can imagine a situation in which the whole point of the story is the not knowing, the never finding out. Perhaps such a story could serve as a metaphor for life, with the wall representing death. Such a story could be effective even if it breaks the second half of Wilhelm's rule.
I guess my thinking is that there are few absolute "don'ts" in writing. Story telling is not a hard science. It's a discovery.