No. Hot water radiators operate within a “closed” system where there’s little or no corrosion taking place. Flushing these radiators will only cause you to add more water to the system, which will create more corrosion, and so on, and on. Why cause problems?
Steam systems are open to the atmosphere so the radiators do see more corrosion than their hot water brethren. However, cast iron radiators come with their own mud legs, and these can hold many years’ worth of scale and rust.
Take a look at the way your radiators connect to your pipes. Notice how the inlet valve or outlet steam-stopping device is always a bit higher than the bottom of the radiator? This is true even when the valve is installed near the bottom of the radiator. The scale and rust settle into that low-slung “pocket” and stays there.
Keep this in mind if you decide to pitch your one-pipe steam radiators back toward their inlet valves to give you better condensate drainage. Take care not to pitch those old beauties too much. If you do, you just might slosh one-hundred- year’s worth of sludge into that inlet valve. You’ll know you made this mistake by the water hammer that pounds on the pipes and the condensate that squirts from the air vents. That’s when it’s time to flush the radiator.