Not that one is likely to find anyone singing "Blowing in the Wind" at a session, but I think I would probably disagree strongly with Greg Stevens about the minors. Quite a lot of the time I play majors where others prefer minors, but there are some cases when the minor is definitely "right". Try "We shall overcome". It can be done all in majors with some speedy shifts between the subdominant and the dominant, but in comparison with the use of a minor in those passages it does not come close to "right". Of course neither are traditional, and neither are "tunes".

I can't say I have ever found a useful set of chords for The Little Beggarman/Red Haired Boy/Insert other name of choice, and likewise (although it is not traditional) "Paddy McGinty's Goat", but I only play them under protest anyway, since I don't really approve of Irish tunes for morris sides.

John Kirkpatrick's "Jump at the Sun" (also not trad of course) seems to demand minors a lot.