It depends on the instructor. I was taught to post first, but I took my earliest lessons on some very rough-trotting Morgans. I got to the point where I would automatically post, and my sitting trot was just terrible/nonexistent. When I got to sitting trot, my instructor literally took the saddle away and had me ride bareback until the sitting trot clicked. I think I learned to post bareback before that happened...
If I were teaching a new rider, I would probably lean towards posting before sitting, just because new riders are usually too tense/don't have the right muscles built up yet to sit without bouncing. Relaxation and muscle will come with time, and better to save the horse's back than to push the sitting trot too fast on horse or rider.