It would be better to shift to the small ring prior to when you will be using it. So basically once you can hold a good cadence with the small ring and like in the 13 or 12 rear, go ahead and change up front then you can shift larger in the rear as you need. You can also shift both front and rear at the same time. But when climbing you are better off not shifting because that interrupts your pedaling just when you need the most power