Can your motors be driven at something other than their nominal voltage?

Yes. In fact, if you can design your device to run the motor slower (lower than nominal voltage) this is a very good thing. Running at lower voltages (and therefore lower speeds) means less brush bounce and less brush/commutator wear for brush type motors, lower current consumption, and longer motor life. On the other hand, if size restrictions and performance requirements demand additional torque and/or speed, overdriving the motor is possible. You must, however, be willing to sacrifice product lifetime if you overdrive the motor.