I trim my own horse's hooves. I learned by watching my friend and having her watch what I was doing. She went to some clinics and regularly meets with her boss's barefoot trimmer to get advice/learn more. I also checked out many online resources and used common sense. When I started trimming my old horse she never moved faster than a walk in the pasture. After a trim or maybe two she was cantering in the pasture. My gelding is generally ridden several times per week anywhere from 30 mins to 3 hours and has not had any lameness issues associated with the trim. He did slip and take a bad step early this summer when I was riding him. He was fighting the bitless bridle I was trying out (stupid stupid me) and not paying attn to what was going on with his feet and strained something in his stifle.
Anyways back to the point. During the summer I found a certified farrier that was trained in barefoot trimming also and he did a couple of trims on Soda. He said I was doing a great job and gave me a couple pointers.
So I don't see a problem with doing it...IF you are going to be smart about it. Also, I would never attempt to trim a horse with leg problems or bad feet or even try to "fix" long term bad trimming.