It depends on the song & how one perceives it. Some can be optional, how you are feeling at the time [cf my comment a bit back about asking Ewan MacColl about Eppie Morrie]. The Anti-War thread has just put me back in mind of Eric Bogle's masterpiece, which, like all of us, I used to accompany when it first came out. But I have been trying it, just to myself, unaccompanied - even with the occasional spoken bit, cante-fable style: & have come to think that un- is really the way that one should be sung.