Association between the jaw - Temporo-Mandibular joint or TMJ - and the neck has always been fairly well accepted. A recent study confirmed the potential benefits that practitioners can have from assessing the TMJ when neck represents patient's initial complain.
Mandible and tongue position alters upper but not lower cervical mobility. Conscious occlusion can also reduce upper cervical mobility.
It seems interesting to control mandible rest position during upper cervical evaluation.
Source : Grondin, F., Hall, T., & von Piekartz, H. (2016). Does altered mandibular position and dental occlusion influence upper cervical movement: A cross–sectional study in asymptomatic people.Manual Therapy. doi:10.1016/j.math.2016.06.007