I always start by finding a tuning which suits the melody and accompaniment of the song, then if my vocals are stretched too far I capo to where the banjo fits in with what I CAN sing.
Certain styles of singing emphasize particular ranges of note when compared to your "normal" range. One traditional technique to sing in a higher key was to use the nasal cavities for resonance rather than singing from the chest and throat.
The reason that Pete Seeger used a long neck five-string (three extra frets) was so that he could drop the accompaniment to a key which suited his vocals without using a different "tuning".
Quack!
Geoff the Duck.