That would be Maybelle Carter, and she typically tuned down as much as five frets (she must have used bridge cables for stringe, even with the big ol' Gibson L-5). She used C position, G position, and even F position, tuned down to match voices and the autoharp (which Sara played, when they had autoharp in the ensemble).

As for those who tune down one fret, I'm not sure what you get from it, other than matching the voice, but rock musicians these days do it a lot -- tune to Eb instead of E. Of course, an electric guitar has less need for significant string tension, since the string doesn't have to move very much mass (other than overcoming the eddy currents in the magnetic field, which is why rock guitarists are so touchy about pickup height), but some claim an increase in "pleasant sounds" from a tuned-down guitar. I'm not convinced, but then, so much of today's electric music has no "pleasant sound" in it that I may be biased.

Bob Clayton