What about when you're holding the horse and it refuses to stand still so you jerk on the reins?
That, I think, really depends on the horse and the particular incident. Is it a horse that is young or afraid? Is he a well trained horse that could be in pain? Is he just being a turd and trying to get next to his friend? Those are all questions to consider in a situation like that. If the horse is young, he may not know that he is supposed to stand still. If he is afraid, his flight instinct may be causing his movement. If he is in pain, movement may help make him more comfortable. If it is any of those, then jerking on the reins will not help and may only make it worse. However; if he is just being a turd and knows what is expected of him, jerking lightly on the reins just enough to get his attention can correct the problem.
I may be a little more harsh with my horses than some people and I am sure that there are lots of people that would say that I abuse my horses because I do give them a pop or a yank every now and then when they need it. A little snippet of advise that my Dad gave me that he learned from decades of working with horses many of which were considered "problem" horses.
"If the horse is afraid, then work with him and show him that he has nothing to fear from you and he can depend on you for support. Never punish a horse for spooking or being scared because it will only make the problem worse. If the horse is aggressive or spoiled, often there will come a point when the only choice is you or him. Many times, that means hurting him before he hurts you. Although, once he stops the undesirable action, then you must stop the reprimand."
Sometimes, you do have to show a horse that you are still the boss and he is a subordinate. I use the same principle with my horses that I do with inmates. "I will give you orders and if you refuse to follow those orders, I will use the minimum amount of force necessary to regain your compliance." Of course, that only works if the horse knows what you are asking and is just ignoring you or trying to dominate you.