On my bigger mare, I always sit because she has a smooth enough trot.
The only time I am able to sit the trot on my pony is when she's going at a barely-moving crawl. Otherwise, it's so horrifically bouncy so I kind of have to post. I'm getting much better at sitting on her the more I ride her, but if I want a good, steady, ground-eating trot, I need to post.
Oh, yeah, and I ride almost exclusively western.
When I first got into riding, my English friend would always tell me: "No, you can't post western". She said it as though it was physically impossible. Well, as I got a bit more experienced, I was able to argue my case and she kind of backed down. She's a very proper show rider, so she does not believe in posting in a western saddle, but for comfort and convenience on my part, yes, I will post. I'm a trail rider-I don't need to do it proper or pretty, I just need to get it done.