Do I have to keep a pantry inventory?
No. A one-minute setup marks what you usually keep around. After that, ticking things off in the store is the only upkeep. Staples stay off your list until you tap Running low, and things you buy sometimes come back on the list about two weeks after you last bought them.
Updated
On the Grocery tab, items your pantry covers sit under In your pantry, away from what you need to buy. Tap Running low on any of them to put it back on the list.