If you choose to do so. I never power mine down. I just turn the screen off and plug it into the charger. It won't affect it one way or the other if you turn it off. It just takes a bit longer to boot up.
Everyones different I guess. I do a restart probably once a month. But I do close my apps completely regularly. From the home screen do the four finger swipe up or double press the home button to reveal your multi task bar. These are all the apps that you have opened at one time. Long press the app until it jiggles and a red minus sign will appear above it. Press the minus sign, this will close the app, not delete it, now press the home button or just touch your screen to return to your homescreen.
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