In theory, a higher key will make a tune (or song) 'brighter,' and a lower key will 'darken' it. Thus, given any song and the mood you want to create with it, there is theoretically an 'ideal' key. That said, if I can't possibly sing it in the 'ideal' key, there's gonna be no mood created at all, so I have no hesitation in changing keys to suit my voice. And I've even had church organists transpose classical pieces for me when they thought I would do the music more justice in a more comfortable key. Agreed that the larger the group you're singing/playing with, the more difficult it is to get them to perform in 'your' key, but, as Don said, there's probably no 'magical' key for any folk music except the one that's best for you at the time.