I always sing eyes open.

I watch the audience. If they look bored you are simply audible wallpaper so get off. If they are really with you then you know it and you can enjoy their pleasure. Trust me it is a transitory moment. Audiences dont seem to get upset with singers with closed eyes but if even one is embarrassed then its one too many. And finally if the audience move towards you its something you really need to know.

By the way a finger in the ear is a technical device to allow the singer to do something clever. I did it once and felt a real folkie. It worked too, I could'nt hear the noise from the rest of the pub.

The Fantum