In the end dots are only a guide, two people can read the same dots and come out with a different interpretation of the tune.
I am with Black Diamond, I bash the tune out on the piano from the dots, put it in my head and then try to play or sing it. Harmonies for me I think come from learning to sing parts in a choir so you then learn to hear what fits. I am only a 21st century folkie so sometime I have to learn it from a book.
I think both skills are useful so long as you are not tied to one or the other.
Steve quotes I think it was BB King who when asked if he could read music answered "Not enough to hurt my playing".