The only reason you would drive the motor over its 6V rating (which will result in its 1.2A limit) is if you want more speed from the motor.
If you do drive it over its 6V rating them you must prevent it from passing its 1.2A limit. You can do this with a "chopper" driver or by adding high wattage resistors on the wires to the coils of the motor.
Take note that the chips on your Adafruit board are limited to 1.2A. If you have not blown the chips already then it will happen soon.http://www.pololu.com/catalog/product/1182
is a very popular driver and has current limiting capability. Your stepper has 6 wires so it can be driven in a bipolar mode as this driver requires.