Do stepdown transformers use their maximum wattage while in use?
The power drawn by a transformer from the wall is equal to the power drawn by its load, plus the losses of the transformer itself.

With a well designed transformer, these losses will be a small fraction of rated throughput. Some losses (core hysteresis and core eddy currents) are constant regardless of transformer load, that is, they're dependent on the transformer input voltage. Copper losses are dependent on the transformer output current.

Generally, transformers are designed to be fairly efficient off-load, you would not expect losses to exceed 1% of rated load. Losses at full load may be several percent.