Answer: Absolutely. Your clinicians shouldn't just document services only to support billing and coding.
The therapist's notes should always clearly indicate every treatment he provides, whether or not the service is billable.
You are correct when you say that the six minutes is unbillable - by Medicare standards, the therapist must spend at least eight minutes providing a therapy service before you can report one unit of timed codes such as 97035 (Application of a modality to one or more areas; ultrasound, each 15 minutes).
But Medicare guidelines also remind therapists to document everything, stating, "A therapist or therapy assistant should record the total treatment time (or the actual beginning or ending time of treatment) for services described by timed codes, untimed codes, and unattended (billed and unbilled) activities."