I have heard from some teachers that one should never let C7 touch the floor in shoulderstand. I am unsure of the reason or how to accomplish this. Secondly, what are your recommendations regarding use of supportive blankets under the shoulders to provide less stretch on the posterior neck muscles? I worry about the decreased stability when one is using the blankets as well as the risk of promoting a reverse cervical curvature. 
I agree that C7 should optimally be elevated durign pose both half and full. However to achieve that it is necessary to have very strong middle trapezius and latissimus muscles, those which abduct shoulderblades and extend the shoulders respectively. To tone them i recommend doing cat bow (push ups) as described in my Structural Yoga Therapy book, page 179. The use of blankets does not create this strength it only acts to make up for these muscles being weakened and their antagonists, the pectorals and anterior deltoids being tight. With proper progressive tone to the muscles i have mentioned their antagonists will have a mild stretch and cervical natural curve can be retained even in the full posture. namaste mukunda