I am not a "fence master", but have built a number of stretches over the years. FOR MY SOIL, I learned the hard way that you must concrete in the posts. I do not concrete in any *single* line posts (use a 4" every 3-4 T-posts), but if I include a bracer (2x4" with cross brace) in the line, I use some concrete. Of course, for the Corners... all are in concrete. After all the work to mark, drill, clean out the holes, I add some rock/gravel to the bottom prior to concreting them in.
If / When I did not do this, over a couple years, even the corners would move/lean. The clay soil I have is terrible in spots and swells / dries out and would move an 8" corner post without any concrete - and a MUST is get below the freeze line or the freeze thaw will push the post out.
It is a pain to dig the holes out after using the auger, add gravel to the bottom (making sure its below frost line) and then concrete. BUT, what is *way* worse is having to re-do it !