I work on a 3 mile tourist railroad (basically a big loop) that contains flat spots, downhill grades as well as uphill spots (~2.5% at the steepest). Depending on which of the three steam locomotives I'm firing on a particular day, how the engineer running the locomotive that day is working the engine, and how many cars we are pulling are usually the factors in deciding how much I can sit while firing. I usually have to fire our smallest locomotive more often (all three are hand fired coal burners) due to its small grate area, while the other two larger locomotives don't usually require as much effort on my part to keep the pressure up.
Our goal is to keep an even clean fire, while also spending as much time in the seat as possible to watch out for any obstructions that are in the engineers blind spot when going around curves.
A twist to the above is when I'm firing our Mason 0-6-4T. The articulated design forced them to locate the firebox pretty much entirely in the cab. We have fold down seats that are functional (though you're crammed in between the cab wall and the firebox), but it's usually easier for the fireman to stand due to the locations of the various appliances in the cab. For example, the injectors, pressure gauge, blower valve and boiler blowdown handles are located near the front cab walls, while obviously the fire door, damper levers, try-cocks and sight glass are located near the rear of the cab on the back head. The downside to firing that locomotive without sitting all day, is that I have the tendency to overfire late in the afternoon when I start getting tired. At that point I usually pop the seat up to force myself into letting the coal ignite and burn properly before firing again.