If you are a kid, and your parents have their own rule about wearing a helmet, then you should obey them and wear one.
Otherwise, it's simply up to you. I think skaters should always wear helmets - I hate hearing horror stories about skaters who get killed or severely injured just because they wanted to keep their hair cool!
But honestly, it's up to you. But here's the truth - you are going to crash. You just will. It's part of skateboarding. And if you crash while doing something fun like jumping stairs or grinding a handrail, there's a very very good chance that you'll hit your head. Basically, you are carrying around a watermelon on top of your shoulders, and all the most important parts of who you are are kept inside. Not a good place to keep your brain when you think about it, but that's where it is!
My advice is that any time you have wheels under your feet, you should have a helmet on your head. And now, here are a couple of horror stories about guys skating without helmets ... these stories are both true.
A couple of years ago I was watching the Free Flow Tour finals, and there was a young guy who was tearing up the course. Like around half of the skaters there, he wasn't wearing a helmet. He was doing well, and then fell from a backside railslide, and smacked his head hard on the plywood ramp. The crowd hushed as medics rushed out. After a few minutes, they got him up - he survived, but was done with skating for the day. The guy had a good chance at winning, if only he'd been wearing a simple cheap helmet...
Or how about a worse story. There was a young new high school teacher here in my town, fresh out of college. He'd been skating for years, and loved it. One early morning, before school, he went to the skatepark to have some fun. The news reports were fuzzy on what trick he was trying when he fell - all we know for sure is, he fell and hit his head. Hard. He was in a coma for a while, and then in a vegetative state last I heard. It's a horrible, sad story, and he likely would still be skating if he'd just had on a helmet.
I know those stories are harsh. I'm trying to scare you into wearing a helmet! But more than that I just want you to understand the possible consequences of choosing NOT to wear a helmet. It's your choice. If you want more encouragement, check out How Has a Helmet Saved Your Life? - you can read other people's stories, and share your own!