It's totally your choice whether you want to or not but I highly recommend it in the park with rails, boxes, bailing hard on jumps, etc. it really saves you!
I can't count how many times I'm hiking a rail or waiting to drop and a guy with a helmet falls really bad and smacks his head on the back of the rail and all I'm thinking is "if that dude didn't have a helmet..." I've had my own firsthand experiences too but just my two cents.
Outside the park there's plenty of hazards out there so again it's recommended but I usually don't wear a helmet outside of the park but I'm taking my own risk.
Yeah, I'm thinking that when in the park I'll wear one or I won't try any rails outside the park. On a normal run I'm not too worried about being run into or hitting any rocks so I'll stay free in that circumstance since I've never had any issues with trees or rocks - although the base of my board might tell ya different: gotta love those exposed rocks in the worst places, lol.
Interesting to hear the different responses, but I'm a bit shocked at the responses towards wearing a helmet. In my time ripping down double blacks at kicking horse, sunshine and castle mountain, I found that most riders didn't wear a helmet. Should note that I NEVER ventured into any park terrain areas so can't comment on that, but sounds like it's exactly opposite.
Tell the truth, even if it leads to your death. - For the truth will set you free.