Don't use the neck bone except for hair that actually touches the sides of the neck. For those back areas, do a smooth transition from head to spine2 - no neck assignments.
The bottom part of the hair you might want to use a little bit of spine1.
Spine2, spine1, and spine0 are all bones in the back. Spine2 is between the shoulders, spine1 is midback (around the bra strap level) and spine0 is around the waist, if I recall correctly.
Smooth transitions from one bone to another (like going 70/30, 60/40, 50/50, 40/60, 70/30, etc. as you move from one row to another closer and then further away from a hair joint) will get you the smoothest animation.