Originally Posted by: Igcuimhne81What I notice is that the instructor is able to leave his thumb peeking over the top of the neck when playing the D string. I find this impossible when keeping my index on the D string 2nd fret and trying to hit the D string 4th fret with my ring finger - I hope this makes sense!![/quote]
It does make sense. But, you don't have to & shouldn't play it that way anyway. :)

I do that sometimes in the videos as a result of trying to keep my guitar neck parallel to the camera viewpoint for the benefit of the student.

But normally I keep the neck tilted up for more efficient posture. And that lends itself to keeping my thumb behind the neck where it ought to be for normal playing!

The only time you should have your thumb hanging out on top of the neck is when it can help you play certain notes or chords as they happen. Then you should return to normal thumb behind the neck position.

One such place to use your thumb on top of the neck is when you bend strings. This happens often in blues tunes. So, I have a tendency to put my thumb up there anytime I am playing anything remotely bluesy. :p
[QUOTE=Igcuimhne81]What I really need to know is my finger positioning a bad habit? Or is it simply an alternative way that needs practice?

Keep your opposable thumb behind the neck to provide pressure that opposes your fingers. Unless & until it hinders your ability to play some technique that requires hanging your thumb up on the neck (for example, string bending).

Hope this helps. Please ask more if necessary to clarify anything. In the future, please know that you can ask me questions directly in my instructor forum:

http://www.guitartricks.com/forum/forumdisplay.php?f=36

Best of success. :)