Captaincranky

I'm gonna say you should probably have your thumb closer to where it was when playing open chords and single lines, and in the middle of the back of the neck when playing barre chords. It's not really absolutely one way or the other, you tailor it to the situation.

The reason it is suggested for a player to have the thumb in the center of the neck, is to enable him to attain enough curve in the fingers, in order for them to point straight down at the string being fretted.

The higher the thumb is to the top of the neck, the flatter the fingers will lay across the fret board.

What happens next is, you start accidentally start muting strings adjacent to the one being fretted. This gets most critical in finger style work, because each note is more important to the entirety of the song, than with strumming styles which oftentimes can have a string not sounding, and it doesn't matter, given that you're often playing chord voicings which use all 6 strings. (You know, 5 outta 6 ain't bad, like that ).

With that said, the thumb position is also dependent of the size of your hands, and/or the length of your fingers. Nobody has he same hands as you, and you need to pay attention to get them to work in your favor.