I agree that when singing unaccompanied, the pitch tends to change over the course of a song. I don't think that cupping the ear makes much difference though - it's a very gradual proces, and the change is no more obvious with the "finger in ear".

As for sitting on a chair backwards and cupping your ear, if you are Ewan MacColl you can do it - after all it was his style and he invented it. Anyone else doing it in a folk club looks like a prat.