If you want to supply 7.2V to the motor, you should use a power supply that can supply at least that much voltage. You should also make sure that the power supply can supply at least the stall current of your motor.
One of the advantages of using a battery with higher voltage is that you can get additional torque and speed. However, using a brushed DC motor at a higher voltage also shortens the lifespan of the motor. If you do use a higher voltage power supply, you can avoid shortening the lifespan of the motor by lowering the duty cycle of the PWM, effectively limiting the voltage applied to the motor.