I may or may not, depending on the horse. Every helmet I had ever tried on has bothered me...It fits right, it looks great, but I hate having something ontop of my head, making me hot, cutting off my periphs, and I feel claustrophobic.
If I'm riding a horse I know really well and trust, I won't wear one. If its a young, untested, new, or all-around crazy/stupid/freaky/hot/hyper etc horse, I will wear one.
However I do not wear one when I am showing. Especially in things like reining, pleasure etc. I feel like if I'm wearing a helmet, that's just one more thing for the judge to notice about me that's differant. Also why I learned to ride onehanded with my left hand instead of my right LOL. Nothing to distract the judge from my horse, or make us anymore eye-catching unless its a GOOD kind of eye-catching ^^
I wear a helmet every single time I'm on a horse. I even wear one out West when I visit the ranch I go to, and I'm the only one who wears a helmet out there. I feel very strongly that no matter how well you know your horses, they're still animals. You can't anticipate everything, no matter how hard you try. I'd rather wear a helmet and enjoy riding for as many years as possible.