I used to never wear a helmet when I rode unless it was when I was taking lessons and it was required. The barn where the horses I have now were required helmets, so I always wore one when I rode. Now that they're here I still wear one. I've taken a couple good spills off my Mustang and didn't hit my head, but since we are more of a rock farm than anything else (we are in NH after all!) I figure my bare head possibly bouncing off one is well worth wearing a helmet. My guy is pretty mellow and never bucks or anything, but even Mr. Lazyhorse could possibly spook at some sudden "monster" popping out of somewhere, and horses do occasionally trip.
Will I wear one every single time I'm riding? Probably not. I'm sure there will be times when I don't put one on because I forgot or am being lazy, but I try to make a habit of it. Will the kids wear helmets? Absolutely. As will anyone who rides on our property, for liability reasons.
I am a stickler for helmets, tho. I don't like the new ones... they look too much like bike helmets. I ride Western, but I wear a traditional black velvet English helmet. Doesn't fit my Western tack, jeans & cowboy boots look, but hey, neither would a wheelchair!