The datasheet shows the "Ohm law" relation - ie what voltage will reach maximum current. What one would like to know is the voltage the insulation of the windings can tolerate. A "modern" stepper driver chip does PWM to keep the current at the desired maximum level but pulses it with the supplied voltage which is several times higher. In theory the higher voltage will allow you to go for (slightly) higher max step speed and give a little more torque.
I suppose you can just turn the voltage up until it starts smoking, then use less than that on your next motor 
Alternativly use a conservative voltage (say 9V in your case) and if that is enough speed/torque for what you final machine is to do, fine. Otherwise beef it up until the motor or your courage gives up. You driver chip will propably limit you to 30V.