This answer relates to my field (business) and it might be different for other fields.
The guideline I have always followed, and recommend to all of my students, is that you should consider the overall "flow" of the document. That is, if you are discussing the results and what is important is the results, then focus on the results. Of course, the methodology would be included but any kind of back-story would not belong in the text because it would distract from what you are really trying to communicate. You want to avoid distraction to keep the reader's mind on topic and that is why we have appendixes.
The back-story may, or may not, belong in the appendix. You should not assume that anything that does not go in the body goes in the appendix. Some information simply does not belong there at all (for example, your musings about the secret connections between Plato's early work and the shape of a doughnut).
The only things which should appear in the appendix are things which are referenced in the main text. If there was no easy place to refer to it in the main text, that is a sign that you should not put it in the appendix either.