Always
I always wear one and feel something is definitely wrong if I start out without it. Same feeling I get when I start driving and my seatbelt isn't fastened. As someone said, you may think to yourself, "I am a safe rider and am in control and won't crash", but so many things are out of your control. As for only riding on a trail being an argument for not wearing one? I have gone down twice in the past five years and one of those times was on a paved trail. I bent my fingers all the way back and needed therapy for months, punctured my leg in the upper thigh, and avoided any cranial injury due to my noggin protection. The other time I went down there was no cause other than a slight lack of concentration and alertness when I didn't see a small hole in the road hidden in the shadows. Tire went in, hands went off handlebars, started falling forward and couldn't regain control or return to upright position. Road rash. Again, helmet was scraped up, but that was it. I see people riding without them and if they are coming the other way as I ride instead of waving I point to my helmet.