Which locations are worth using a pharros stone? [closed]

I already encountered the pharros slot in the Wharf (which I found quite useful), the one in Earthen Peak (which wasn't so useful) as well as the massive amounts of pharros slots in the Grave of Saints.

In what locations is the use of a pharros stone tied to a welcome result? Getting to know the game, I don't assume to be able to use all slots in one playthrough...

By "welcome result" I aim for results like exposing items, leading to hidden paths or decreasing the difficulty of the game (like in the Wharf).

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Where should I use my Pharros Lockstones?

Usage

Forest of Fallen Giants: Shows fake wall in room under ballista trap near Mild-mannered Pate in the Forest of Fallen Giants - Cardinal Tower bonfire. ...
No-Man's Wharf: On the second level area, in front of the house. ...
Lost Bastille: ...
Earthen Peak: ...
Iron Keep: ...
Doors of Pharros: ...
Grave of Saints: ...
Shrine of Amana:

What Doors should I open in Doors of Pharros?

1st Room (the one you start on) left side: First and second door have Gyrm Warriors (one each), the third has a Rat. Big Stone Door: The 3 locks each opens one side of the door, you only need to open the one in front, underwater. Contains a Primal Knight and a chest holding the Santier's Spear.

How many Pharros Lockstones do you need?

22 in Doors of Pharros, only four of which are necessary to collect all treasure, as only the middle section of the large doors must be opened for the player to fit.

All Pharros' Contraptions Locations: Dark Souls II: Scholar of the First Sin

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