Since at training level you can sit or post the rider should select which way the horse best responds. If the rider can sit effectively and not block the nice forward motion of the horse, then tyhe rider should sit. If the horse is green then normally the rider will post since the horses back is generally weaker until they build up a topline.
For a well schooled rider and not quite so green horse I suggest sitting most trots and post the lengthening (First level), but for Training level I would post the trot MOST of the time.