Bad idea for anyone with a neck injury. The time to practice neck mobility is *off* the bike, preferably someplace with a soft place to land. For years after my C2 injury an instinctive peek over my shoulder while driving, walking, etc., would cause an electric jolt-like surge of pain, accompanied by dizziness and nausea. It took years of healing and physical therapy to regain enough flexibility to get something close to a normal range of motion without pain and dizziness.

Even now I still do neck stretches before taking off on the bike to be sure I can turn and look over either shoulder without pain and dizziness. I don't take anything for granted. A sudden neck spasm could be disastrous.