I haven't poled in many years, but I never wore a helmet when I did. I would only consider wearing a helmet in river rapids for which I would wear a helmet as an open canoeist.
Decked boaters will roll under the boat if they dump, so they should always wear helmets. Open boaters are less likely to hit head first if they dump, but I have seen one cut head on an open boater who was likely thigh-strapped in. I would think a standing poler is probably even less likely to hit head first than a sitting/kneeling open boater in a spill, but if it did happen it could be with more force.
Helmets inarguably are safer, and also provide head warmth in cold weather, but depending on the type of waters being poled, safety is not always the dominant criterion.
These polers pole unhelmeted while balancing on bamboo logs:
The members of this forum are so skilled that I'm sure if they began to fall out while holding a pole, they would avoid injury by simply doing this: