I ride both English and Western. It's true that posting will get you disqualified in western shows, but many western trail riders post on trail. It's much easier on the horse's back over long distances. My last riding instructor (who is the head western riding instructor for the USA's top Equine Science University) told us to post when we are warming up horses at a working jog. This is so much easier on their backs. I also worked at a horse camp for the past two years and we taught the kids to post in a western saddle because it's just more comfortable for some people, especially on trail. With some of the horses and ponies I rode there it was nearly impossible to sit the trot on them, WAY too bouncy. And some horses would not trot until I started posting. That's just the temperamental nature of camp horses :\.
So no worries, lots of people do it. I think more people should. If people are giving you funny looks, pay them no mind. If you and your horse are comfortable that's all that matters.