When I was an active guitar teacher, I told my students to look to the position to determine where the thumb had to be. Barre chords are "thumb-behind" chords, while most other chords are "thumb-on-top" chords. That D chord (with the thumb on the 2d string, not the third, BTW) or even the "thumb G," the first chord my uncle ever taught me (3X0003, 6th to 1st). I have even made the movable B7 chord with the thumb over the top on both 5th & 6th, and the index finger available for the 2d string (so c7 would be:

T3
T3
M2
R3
I1
P3

I hope that's legible.

So when I play, I use the thumb on the edge of the neck most of the time, but slide it around to the back of the neck for barre chords. By the way, when doing these chords, it helps to move the thumb bridge-wards from the barring finger, so it's behind the 2d or 3d finger. This rolls the index finger slightly over onto its edge, which is better-equipped to fret strings. The fleshy pads on the "front" of the finger aren't so good at fretting as the edge of the finger.

As for anchoring the right hand, for most steel-string players, it's necessary, simply to get enough "oomph" in the picking motion. You don't get much volume with free-strokes (classical players will probably agree that loud rest strokes are easier than loud free strokes), and the anchor helps with volume. I usually anchor with my ring finger, because using the pinky means putting the hand at a funny angle, and also means restricting the reach of the index and middle fingers.

As a steel-string player, in fact, I have trouble with some of the classical-derived "parlor guitar" patterns that can sound so "right" on old 19th-Century songs. All those old patterns from the Jerry Silverman books ("Folksingers' Guitar Guide" et al) are harder for me because of the anchoring habit, which is why I mostly stick to John Hurt or Merle Travis for my models.

Which brings up the subject of how technique guides style, but that's another thread, for sure.

Bob Clayton