Gee, I let my horses graze all the time and none of them have bad habits
They all respect the fact that when I say time to go, it's really time to move on. They have great manners, even if I walk them across a REALLY good bunch of grass.
While teaching my new horse to respect my asking him to come along, he had the attitude of "Whatever, I'm ignoring you." So I put a feel on the lead rope, not pulling or jerking, just a steady feel, and if he didn't respond within a reasonable amount of time I tagged him with my stick on the butt, then walked on as if nothing happened. I gave him plenty of time to respond and before I tagged him I made it VERY obvious it was coming. If I don't have a stick, I use my lead rope and I'll throw it behind his drive line to cause him to go forward.
Now I have no problems at all. I never once jerked on him, yelled at him, or got aggressive. It's just not needed to correct an issue like this. It's actually very simple to fix.
One thing you may want to consider.......ususally when people deem the horse NEVER to graze, either while leading or riding, it's usually those horses who fight the handler so much for the grass. If you use the grass as incentive, or at least allow the horse to graze some as long as you have the communication established so he knows to come along when you ask, then your problems will be greatly diminished.
Like I said, I always let my horses graze and I don't have any problems with them. If they give me what I ask, then it's only right to give back to them.