It seems to me this question is a fairly new one, historically...and has come to pass because singer-songwriters are the in thing right now. But going back to the music of the pre-60's, there was no question that once the song was written and "out there" it was up for grabs for however any group/band/song bird/singer wanted to do it. Very often the definitive version was several iterations beyond the original.

I can't imagine doing a song exactly like someone else...as someone stated above, why not just play the record and get the real thing, if that's the case. I don't even consider playing a song for anyone if I haven't committed it to memory and played and sung it until it has become a part of myself...which, by necessity, means it's going to be how I do it, not the writer's (except that we all copy a little from here and there, now and then, often without even knowing it).

It may be heresy to say it, but to my ear that are many fine songwriters with less than pleasing voices, for instance, and I am thrilled to hear their execellent music performed with an instrument/voice that does it justice. Of course, the writer should be given credit, and if major changes are made, responsibility for them taken.