You should document consent every time you take a photo.
There has been a slew of information written on the topic and it varies between counties, regions and hospitals. In some countries there are fairly prescriptive rules stating consent must be obtained while others leave it open ended.
Doctors often don't get consent, and when they do, they don't record it properly. A whopping 82% of the time, doctors don't document consent when taking a photo.
A study among dermatologists revealed that only 2% obtained written consent! While 46% received verbal consent, they failed to document this. A further 30% acquired verbal consent and documented it in the clinical notes, but they did not record it using an appropriate consent form.
We suggest you always obtain consent. The ease of distribution of digital images, the difficulty of properly de-identifying photos, and the grey area around when consent is required, in practice, means consent should be sought to cover yourself. Being able to point to documented consent may save grief should dispute arise in the future.