ALWAYS. I'm not a lawyer by any means, but there are a lot of things that can go wrong on trail rides, especially when you throw people who aren't educated about horses into the mix. In addition to accident and liability insurance, make sure you have a strict list of rules that guests have to adhere to if they want to go on trail rides.
Along with the normal stuff like appropriate clothing, shoes, and head protection (trust me, you WANT to enforce a helmet rule for your own legal protection), I suggest a list of barn rules. Include common sense things like "no running" and the like, but also include things that we horse people would know not to do, but that the average person might not--like don't open a can of soda you took with you in your bag on the trail ride while still sitting on your horse.
If you don't already, make sure your guests sign liability waivers. When I went to college, even though I was there for the equine program, had been riding my whole life, and completely understood the risks, I still had to sign a liability waiver and a helmet waiver.
I suggest you not only get insurance through an ag or equine insurance agency (so they understand exactly what your needs are), also talk to a lawyer who practices ag or equine law to draw up the appropriate paperwork. The more you can protect yourself and your guests legally, the safer you are if someone gets hurt.