When playing the "A" section(the main theme) of Davy Graham's "Angi", I don't use the left hand thumb to play the 6th string 1st fret - as would Davy(I think) or Bert Jansch. This is undoutedly because I use a classical guitar with a widish neck. However, using this technique does open up moves that a "thumb over" player might not get in to.