The patient's head should be upright and unsupported during rest position recording to ensure a natural, relaxed posture. This position allows for accurate assessment of muscle tone and mandibular rest position without external influence. Supporting the head or tilting it alters muscular activity and jaw alignment, leading to inaccurate measurements. An unsupported, upright head mimics functional conditions, providing reliable data for diagnosis and treatment planning in prosthodontics or orthodontics.

Reference: Contemporary Fixed Prosthodontics, Rosenstiel, Edition 5.