Well, since I don't know the song, I obviously qualify as an expert. My understanding of the "salt over the shoulder" is that people believed they had a good spirit sitting on (or behind) their right shoulder and an evil sprit on the left. Since salt was considered a luxury (cf. the word "salary" -- Roman soldiers were paid in salt), spilling it was a bad thing to do. It was thus necessary to appease the bad spirit by offering him/her/it some of the valuable salt.

Of course, that begs the question of why the bad spirit was on the left and not on the right. I suspect that people in times past might have felt that there was "something wrong" with their non-dominant (usually, left) hand. It looks just as strong and healthy as the other one, but it just doesn't work the same way. It wouldn't be surprising if they thought there might be some bad spirit preventing that hand from doing what it might otherwise be capable of. Hence, we have the "evil" connotation of the word "sinister", which in its original literal meaning simply refers to the left side.

It would then make sense, if one were coining a phrase with a somewhat "sinister" tone, to choose left over right, if both were equally applicable. That is to say, the judge seems a bit more threatening if he's looking over his left shoulder rather than his right. It's a small "toss-off" point, but that's what good folk songs and poems are made from.

Thank you, Mudcat, for giving us closet polymaths a forum for trotting out these wonderful theories of life, the universe, and everything!

Aloha,
Mark