In some maps, you will find yourself with multiple keys and the doors where to use it, just to discover later on a door locked with a key from a previous map.
What are the keys it makes sense to keep and for which map, at least once?
The game has 11 different keys:
You can use one gardener's key each in the ramparts, the graveyard, and the forgotten sepulcher to collect the moonflower keys. To take a route through all three of those places you need to have 3 active boss cells.
You need all 3 moonflower keys in high peek castle to unlock the blueprint "Acceptance".
You need the cavern key in the graveyard to open the path to the cavern. You only need to do this once.
The village keys are noteworthy, though not required in a different stage. There are 2 keys and 3 doors in the stilt village. 2 locked doors will block your way towards the end of the stage, where you will find a third door that you need to open to get the blueprint for the counter shield.
You can skip the first door by taking a secret path above it. Doing this allows you to take one of the keys to the third door
Read here for more information on keys.