Since there is no stress in the material, the cast iron bed will not change shape over the years. Steel press bed. Later on, cylinders and beds were changed and made of steel. Steel is much stronger than cast iron, and give no risk of breaking like cast iron does. There is however a great difference in the manufacturing process.
Steel is rolled in huge rolling machines.
This creates enormous stress in the material, especially near the surface. This is the reason that steel, when used for etching press beds, needs to be temperature treated. It remains in an oven for three days at 800-1000 C to loose the stress. Now it can be planed without risk. It shows because, if a bed has not been treated properly, the stress releases itself slowly, over the years.