"The nominal voltage is irrelevant for all practical purposes. The important figure is the rated current. "
I disagree with this statement. the manufacture offers data for the end user to use for calculations. this statement is akin to saying that resistor wattage is irrelevant for all practical purposes in data circuits.
the stepper motor is a coil, the coil has inductance. what you can do with a coil is very much based on manufacture. ergo the manufacture offers test data. the motor power supply calculation requires you know the motor voltage, then allows for the inductive reactance.
a motor has two major causes of heating, copper losses and iron losses. copper losses are from power flowing through the motor. iron losses are from the eddy currents and hysteresis heating.
every motor has a maximum voltage rating. it is a disservice to dismiss a part of the calculations without describing why.
I offer white papers from a stepper motor designer. probably one of the foremost authorities on stepper motors.
You would do well to read and understand how a stepper motor uses voltage and power and why they are so important to understand, especially if you want to post as a source of teaching for others.
http://www.geckodrive.com/app-notes.html
he has a simple writing style and except for a few spelling errors (ration instead of ratio) he is very readable.