Well, the internet has made it much easier to compile songs. In an afternoon I can usually download a half-dozen variations of a song I want to learn. I usually read through them and "sleep" on it before editing them down to a version I want to do.

Like Claire, I find that being totally "complete" means ten or so verses, and that's usually too long, so I almost always eliminate verses that don't carry the story forward significantly.   And, as Claire also states, sometimes there are conflicting plot elements. If there is a valid history to the song I try to hew to the original facts.

Of course, there is also the matter of language. Colorful imagery also makes a memorable song, so some verses make it in the final version just because of that.

I'm sure this kind of collecting and editing sounds offensive to purists, but to my mind it's just this era's high tech condensation of the folk process.