My grandmother said it had to do with throwing a pinch of salt over your shoulder to ward off evil spirits and lies.

You only do that after spilling salt accidentally, and it's to knock the devil off if he happens to be sitting there (which would be what made you spill the salt at all). This, for my English ancestors, was plain common sense, unlike the superstitious balderdash adhered to in benighted lands...

Your grandmother is not an etymologist

I'll have to let her know.