At the risk of making this question seem entirely too complicated, I'll take a stab at it.
Your really asking a couple questions here. When a locomotive is working under light to moderate loads, the Johnson bar is placed as high in the quadrant as the engine will run without sounding off-beat or "lame" and the speed is regulated primarily with the throttle. When the slowing or stopping, the reverse lever is moved forward as the speed and throttle setting decreases.
When a locomotive is drifting and supplying no power to move the train, the Johnson bar is placed in full forward or nearly full forward position. Depending on the practices of the Company, the throttle is completely closed or cracked open only enough to keep a small amount of steam moving through the cylinders to lubricate the valves and cylinders.
There are actually very few places in railroading where one can simply "coast" with throttle shut, and brakes released. Because the the locomotive does not roll as freely as the cars, coasting will cause the slack to run against the locomotive, when the throttle is opened again, or the brakes set, the slack will run out. On passenger trains this puts folks to the floor and spills coffee in the diner. On a freight train this puts the conductor through the cupola window and pulls out drawbars. A lot of ham-handed "hoggers" ran this way much to the displeasure of the crews he worked with and the Road Forman who had to supervise them. In order to avoid this kind of situation, one always sets air on the train first. Once the brakes are set, the throttle is eased off. From here it is a very big judgement call concerning speed, curves, grade, tonnage of train, etc as to how you balance power and brake. Sometimes you set too much air and have to release the air before you stall or stop short. With very long trains (say more than 30 cars) releasing the brakes is an iffy deal as the head end will release way before the rear will and you risk putting too much strain on the head cars and pulling more drawbars. The DRGW lost lots of wood idler flats this way.
Even on steep grades, a small set is needed to keep the train stretched, unless you're working light cars on very steep grades like Mr. Shreve. When we used to run our passenger trains with the box cars, you could successfully power to a stop on the 4%, but once we started running the bigger passenger cars, about 5 lbs of air was needed to keep them from running in.
Now, that I have confused everyone, I'll shut up.
