For me, it's a question of intensity. Very intense songs tell me to close my eyes. "1952 Vincent Black Lightning" = eyes closed. "Smoke, Smoke, Smoke that Cigarette" = eyes open. Also, eyes closed, if a song is new or if I haven't done it in a while and am having to work at remembering the words. Helps to focus the concentration.

I'll bet Doc Watson has never had to address this issue.