You'll wish you wore a helmet when the instructions your brain sends to your muscles take a second instead of being instantaneous. I fell last summer from the top of a 6 ft quarterpipe onto the side of my face/head, and don't remember about 30 minutes afterwords. The next day, I was constantly dropping things, buttoning my shirt was hard as hell, because the messages my brain was sending weren't traveling as fast as they normally do, thanks to that 3rd concussion. Even walking down stairs made me nervous, because I couldn't trust my legs to do what I needed them to.
It was the scariest day of my life. Wear a helmet as often as possible, especially when you're trying new/big things. You'll never regret wearing a helmet, you'll only regret not wearing one.