They can only fire you "at will" if they continually catch you not following the methods, which means they are watching for you to do so. All of the managers and supervisors I have worked with are not inclined to do so; in centers with 60-70 drivers, 50 trips on a light day, they have too much other stuff to worry about. Stay off of their radar, and you eventually recede to the background.
It's why I worry about the "us vs them" mentality promoted by some in here. Maybe it is just my building, but the upper levels of management don't key in on specific centers and look to take heads as much as you guys seem to indicate goes on in your buildings. What that means is, yeah, no one will be perfect in doing the methods everyday, but they aren't that nitpicky, unless you are a hothead who loves to go to battle with management over every little grievance of yours, making them WANT to get rid of you. Even then, it takes a lot to get then to actually get in a car and start following you around to visually observe you "not folding in a mirror", or "not holding a handrail while exiting a vehicle". Usually by that point, you've gotten on their nerves some other way.
I've been disciplined via the progression I've described above, and have been told it by both management and the union. I've seen where some on this board have said, "No one is GIVEN a one-day suspension; discipline is REDUCED to that", and yet I have been given several one days off the bat, most of which are reduced to letters. At the moment I don't have my contract book in front of me, but I believe I've seen more than once the progression in there, and I am mystified when I read why some guys get suspended or terminated for first time things.