5-10 years ago, the owner of Gecko used to post on the forums quite a bit. He used to say that the ideal voltage, was the lowest voltage required to provide the performance that you need. And any excess voltage is wasted energy and heat.

They then came up with a formula that was the theoretical maximum (not "ideal") voltage. As all motors are different, this formula isn't a firm rule, even though their website will lead you to believe that.

Here's the main issue.
In an industrial application, a stepper motor would be sized to have 30%-50% more power than the application required, to insure the motor will never lose steps.

In the DIY CNC world, a motor is selected, and then the user tries to run that motor as fast as he possibly can. This requires throwing more and more voltage at it. With better drives, like AM882's or EM806's, the current reduction, and other features, can allow you to run at much higher voltages, without the motor overheating. Mounting your motor to a big block of aluminum also helps keep the motors cool, by acting like a large heatsink.