I remember a few years ago reading a review of a live concert in the New Musical Express, which stuck in my mind as a folkie. I can't remember the name of the band, but the reviewer said "... the singer was so bad that he had to put his finger in his ear to stay in tune ...".

I agree that "If it helps, do it. If it doesn't, don't", but I still maintain that if your voice is good enough to hold a tune, you can do it with or without the "finger in ear", so perhaps it's more about a feeling of insecurity.

I think it looks better and more professional though if you can avoid it in a live performance.