It depends on your riding style and where you are riding too. If you average 10mph and ride with slow traffic, risk of injury is low. The helmet is probably neutral. Riding fast with fast traffic? A helmet is more beneficial hypothetically.

There's is no right answer. I ride slow on mostly quiet streets. I usually strap a helmet on but if I need to not wear it for some reason (like I just got my hair done) I am not too concerned if I am riding in residential areas. I know where most of the bike collisions happen and I rarely need to travel those routes.