This is an old topic, but I need to resurface it...
Only the motor power pin is used to power the motors.
You should use 5V for 5V motors. That 5V will be applied to both motors (well...minus losses in the driver itself, which can be pretty significant). You will probably get more power out using 10V (or so) and using PWM at 50% duty cycle (or so) to get approximately 5V on average delivered to the motors.
Don't freak out about applying 10V to 5V motors, as long as you don't exceed an AVERAGE voltage of 5V you will be fine (motors are mostly destroyed by current, not voltage). 50% duty cycle X 10V --> 5V average voltage.
To what pin do I apply the PWM set to 50%? To the one of the logic pins?