I'll sing a song as close to the way its written as my sensibilities allow. I really hate "put-on" archaic pronunciations (see: "The Elfin Knight," as sung by Ewan MacColl) or out-of-place colloquialisms (see: "East Virginia" as sung by Joan Baez), so I do my best to avoid them in my own singing, although I will for some reason develop a twang against my best intentions while I'm singing some Woody Guthrie songs. Hopefully I'll outgrow that. In general, I think a folk singer should sing as close to the way he/she talks as possible, while making allowances for the act of singing and the credibility of the song ("And it's a HARRRD RAAAAAIN that's going to fall").