OP, if you are comfortable playing in C, G, D, A and E, then you are never more than a tone away from what might be the ideal vocal key, even without using a capo. Since few folk songs cover very much range (compared to classical art songs) there shouldn't be much of an issue.
Where I use the capo (besides what I call the "fake" drop D mentioned above, which I love) is when I want to play, say, G shapes for the sake of the chord voicing, but need to sing in Bb or C to be in a the right part of my vocal range.
Since I can play in pretty much any key (but much prefer the sharp keys) for me it's a matter of fiddling around to get the sound I want on both guitar and voice. Sometimes it's a tenuous compromise. Sometimes I can't find it, and I just don't sing that song. Or I pass the vocal on to Mrs Highlandman, who sings better anyhow.
-Glenn