The voltage will be relative to the motor’s inductance, so it has a bearing on how fast the motor can react, but overall, what matters is the current rating.
This is because the stepper drivers regulate the current by varying the dury cycle of the power supply to provide effectively lower voltage until the specified amount of current flows.
Therefore what is required is that your power supply voltage is higher than the motor specified voltage. This is slightly complicated by the fact that the motor itself acts as a voltage generator when it turns, which lowers the available voltage to drive the steppers. This basically limits the maximum speed that the stepper can turn, or at least the available torque at the top end speeds.
Tl;dr: Don’t worry about it. The stepper drivers take care of voltage.
If you were to hook up a DC power supply directly to the motor leads and leave it for a while (motor not turning), the rated voltage is the voltage at which it would overheat. Or stated another way, the rated voltage is the voltage that would produce the rated current given the resistance of the windings.
We don’t use the motors this way, so it does not matter. The motor controller maintains the current below the rated current, and that is enough.
The rated voltage matters for unipolar motors when using a ULN2003 for example because you are basically applying DC through the motor and the current is determined by the supply. But for our purposes the voltage rating can be ignored.