There seems to be a real conflict of opinion as to whether this thumb should be pressed against the back of the neck or hooked around it with the flat of the thumb on top of the neck. Most books say to press it against the neck, but some say that although this is the proper way many guitarists use the lazy way. Indeed, most rock guitarists have their thumbs hooked around on top of the neck with the span between the fingers gripping the back of it.
Many people think that the thumb should be grabbing or pressing the guitar but the truth, for me anyway, is that the neck of the guitar rests against the thumb. This is a subtle difference but it is important. By not consciously gripping or pressing, my thumb pretty much follows along with the rest of my hand, almost always a mirror to the index finger on the fretboard. By not gripping or pressing, it is free to glide, not only up and down the neck, but also in its position on the back of the neck. This is essential to the way I play because I have small hands and on a thin neck like on most electric guitars, I will ”barre” certain chords, say an F chord, by gripping the bass note with my thumb, like this:
E - 1st fret (index) B - 1st fret (index) G - 2nd fret (middle) D - 3rd fret (pinky) A - 3rd fret (ring) E - 1st fret (thumb)
When I play a D chord, I will more often than not grab the 2nd fret of the low E (the F#) with my thumb as well if I want to make a final ringing chord out of it.
In both these cases I am using my thumb in a “rock” or hooking position in order to get notes that are otherwise awkward for me.
But frankly, most guitarists grip with their thumbs because they have no choice in the matter, seeing that they often hold their guitars way to low on their body in order to get a good grip. It's ironic because while it looks cool (I guess) it truly limits their ability to play. They are using the thumb to hang on to the guitar, not using it to play. Do you understand this difference?
I think that it's important to understand that your thumb should be more than a prop. It's a player. It either grabs a note or makes it easier for the other fingers to reach theirs. If you worry about getting the notes first and let the thumb fall where it may, I think you'll find that, eight times out of ten, it will end up in what you call the classical position. But there will be times when it will be in the rock position too. The thing is to not let the thumb dictate what the rest of the hand can do. And I also think that if you follow this “note first” philosophy, that the question of comfort won't even enter into the picture. When you leave the thumb to last, it will automatically go for the most comfortable place.
One thing not to lose sight of is that you need both hands to play guitar, and it is extremely important to develop both hands