In very hilly terrain, there are several approaches, depending upon what your goal is. Do you want to proceed in a big hurry, but not drain the battery too much? Do you want to just keep up with traffic and acheive good mpg? Or do you desire absolute max mpg? My drives are usually in the latter category. Here's one method: 1/2 mile prior to the hill or climb, slightly increase speed, maybe by 4 mph? As you ascend, allow the car to slow down as it wants; just keeping enough gas pedal to AVOID using any assist. Then downshift to 4th at 49mph. Then downshift to 3rd at 42 mph. If the hill is really steep, downshift to 2nd at 34mph.
All during this time, downshift smoothly while matching the rpm's. It should be seamless. Try NOT to use assist. It takes patience, and cars will be passing you. Then at the top of the hill, slowly let the car accelerate and when you sense the downhill is steep enough to allow the car to speed up using no power, just shut the engine off (FAS) by turning the key back one click, wait 3 seconds, then turn the key on 1 click (to reboot, engine still off) (be SURE not to go back 2 clicks and lock the steering wheel!) Don't restart the engine again untill your speed is level, and just beginning to decline. (50+ lbs of air in tires REALLY helps the car roll !!)
Practicing and perfecting this method can result in SUPER mpg, and soon it becomes 2nd nature. By NOT using any assist, you actually save fuel by not working the engine to recharge the IMA system.
I must note, however, that if you don't wish to drive this slow or don't have the patience, a more moderate approach is called for. Yes, using assist and recharging, as intended, can increase mpg and performance in a moderate scenario. But just remember: there is no free lunch, UNLESS the downgrade hill is very steep, and you would otherwise need to use your brakes.