When do you spare constrictors in head and neck treatment planning?

In contouring base of tongue, tonsil, larynx and other H+N tumors, the CTV and PTV will frequently encroach on the posterior pharyngeal wall. I'd appreciate any comments or references as to when one can reasonably spare the pharyngeal constrictors without compromising appropriate target dosimetry, and if so, where does one begin to contour them and what are DVH/dose limits? What dose limits do the experts use in the field for sup/mid/inferior constrictors?