I also love back stories, and have often heard or read quite contradictory versions of how a song came about.I would suppose that you'd have to treat a song like any historical document, making sure to approach this process with a certain amount of caution.

Facts are such slippery things and hard to be sure of. It's imperative that you cross check all of your sources, even when there are few source documents available. A lot of "subtexts" and back stories, which seem to shed such a wonderful light, are self-serving bits of revisionist history. If we were to believe Jerry Lee Lewis' accounts, we'd believe he invented rock 'n'roll practically single handed. Inaccuracies get repeated, transliterated, and referenced to the point where one bad commentary is used to prove another, which was in reality it's source. "Lore" can be nothing more than "word dancing" (as in, "boy, could he make those words dance"), whether it's done in reference to the origin of a song or of anything else. But I'm just an amateur student of history; I hope some of the heavy-hitting musicologists weigh in on your question.