So, I haven't really been playing that long and started off trying to keep my thumb in the 'behind and parallel to the neck position'. It's still the way I find myself playing most of the time, especially when playing melodic or lead parts. But then I hit two things:
1. The first lesson that asked for thumb fretting. I think this was probably David's version of Blackbird for the Gm chord, but there are plenty of things that require thumb fretting. I've got a Mel Bay book here, which focuses on traditional and classical repertoire and it has exactly the same fingering (thumbing) for the Gm. At this point I thought "Ah, that's the game then."
2. Barre chords, this is probably the point at which NoteBoat is staring in horror, but it's not as bad as it first sounds...
I've just tried out some stuff looking at what my thumb is doing. When playing single notes and open chords it's right at the back of the neck and upwards (not straight up), then it moves according to where my wrist needs to be. For partial barres which move quickly (e.g. Rock And Roll, Back in Black which I'm playing partially barred with one or two fingers doing fretting below) my thumb moves parallel to the frets, not for pressure, but to act as a pivot and stability point when moving those positions, maybe if I improve my fretting dexterity I wont need this so much. Then going to full barres I find that the first couple of frets do require a little pressure from behind to make them clean, so the thumb is still parallel to the frets for that. In all those cases it's behind the neck, actually for full barres it's further towards the 1st string than most of the time (past the halfway point).
Then there are two GN lessons that get very different treatment: Star of the County Down podcast
and Man on the Moon
. Man on the Moon is played with open chords and a change from C to Dadd2add4, the C chord needs the little finger all the way across the fret board and this gets me into exactly the classical position, which then sticks for the rest of the song since there's no real need to change it. Star of the County Down on the other hand has a thumb fretted 6th string at one
point (D/F in the chorus) and is one of the few songs where I find a lot of the time my thumb is outside the back of the fretboard for much of the time to be ready for that.
As for bends, I seem to do this differently for different types of bends. Trying to learn Samba Pa Ti at the moment and the unison bends in that (e.g. 5th and 7th^2 frets) get done from a classical position, whereas there's a 15th fret bend in Rock and Roll that gets the thumb right round. The difference I think is that the unison bends its a higher fret on the lower string getting bent, this means the fingers form a row down the fretboard. On the other hand at the 15th fret, G is being played on the E string and D on the B string is being bent, to get access and to get the fingers side by side for the bend it's necessary to rotate the wrist.
This beginner's feeling so far is that thumb behind the neck is a good thing to teach because what you want to achieve is mobility. It seems like it's easy to start out with the baseball-bat type grip where the thumb goes round the fretboard which pulls your hand and your fingers behind it, leaving you attempting to fret sideways. With the thumb gripping like that you don't have reach and you get into the habit of gripping the neck which doesn't give you mobility, this is very different from changing your position according to where your fingers and wrist need to be. Starting off with the thumb behind the neck stops you doing that so you can learn to move around.
(Well, that was a longer post than I expected when starting.)