I think if you're singing which is truly "American" in style and context - Jimmie Rodgers' "Waiting For A Train", for example - then an accent which suits that style and context is appropriate. And the same goes for English songs - "The Rout Of The Blues" is an example here - then an English, rather than American accent is most suitable.

I know one good local singer-songwriter who performs his own songs. His natural speaking voice is a mellifluous Scottish one, but the moment he opens his mouth to sing one of his own songs, he sounds like second-hand James Taylor. The effect is to diminish, rather than enhance his work.

Mind you, I'm also mistrustful of those English folk singers who appear to affect a nasal 'Mummerset' accent when singing English songs. Why?