Re: 20 Songs you need to know
This is really a very regional kind of thing. You basically have to go to jams to find out what they are playing to know what you need to learn. Around these parts, songs are king, you can go for months and never play a tune. (Using the Song = words + music and Tune = just music rule.) Lot's of people can do the tunes, like Soldier's Joy, Old Joe Clark, Redwing, etc., etc., they just don't. I know a bunch of tunes just because I like to play them.
My communal effort in songs is learning choruses of a lot of songs so I can do harmony parts. People always want me to do harmony in jams so I do my best to oblidtge. The songs I learn are strictly what appeals to me. I do try to keep them in the 'jammable' range but not always. By learn I mean memorize, I do not carry a big jam book (though I do keep a song list tucked in my tone gard, I remember more songs than I remember I know).
Weber Bighorn Mandolin
Martin D-18 Del McCoury Guitar
Weber Gallatin Mandola