Is a helmet mandatory? No. Is it a good idea? Sure.
Really, it is up to the OP. You don't need one, but what is your comfort level? I rode for 10 years without a helmet. I've been rocking one since 98. Overall, in the cold climate I ride in, it offers more advantages. Warmth, protection, blah, blah, blah. Keep in mind it offers little protection if you are going big. Say if you hit a tree at 20 (easy to achieve) mph. Glancing blows, flyswatter slams, helmets are great protection. So again, it's preference. Of course if you take a flyswatter slam to the head and die, we'll say "we told you so".