Songs have a life of their own. I don't see how they can, or should, be tied down to one version even if they are recent. I'd say the greatest respect that you can show a song is to produce the best version you can, even if this means varying the version you first "captured". The song is more important than the singer or even the composer – if they're good they'll last a lot longer!
I've tried making up some songs myself but I'll sing them different ways depending on where I am and how I feel at the time. If you're going to mean it when you sing a song surely you can't be rigid every time? [If anyone out there is ever foolish enough to sing one of my songs; do it your own way, I'll just be glad someone's singing.]
As for "traditional" songs; if they have come down through oral transmission I think you are doing them a disservice by trying to fix them totally at the moment they were first collected. I'd rather look at a living flower than a pressed specimen any day.