So I perhaps should have taken my worries further. People usually don't know what hand they hold a phone in, and they make assumptions based on their personal handedness. You can't be more accurate than this because people often swap sides. We observed this when people wanted to write something, or reached into a pocket for a notebook, etc. For instance, when they were walking with someone else, they always used the phone on the outside of the pair.
People are also one-eye dominant and use one eye only when asked to line up two objects.