Although, I do have another question pertaining to thumb placement.

When I first taught myself the D-Chord, I learned by placing my thumb in the middle of the neck. I found that position to be comfortble and allowed for adequate finger reach.

Now that I am learning the C-Chord, I am also learning transitioning between chords, specificall between C and D. I noticed while watching the vieo lesson on switching between the C and D chord, Lisa McCormick places her thumb at the top of the neck, muting the low E string while performing the C and D Chord. So when she transitions, she does not have to move her thumb position.

So my question is: should I learned to play the D-Chord with my thumb in the same positon it is when I play the C-Chord; or should I learn to move my thumb position when switching between the C and D Chord?