Neven Mrgan on labeling the Back button in iOS apps:
The Back button should never show the text “Back”. [...]
This is redundant and it provides no context. Note that Apple never does this, not in any app. Instead, they provide either the full title of the previous view, or an abbreviated/truncated version of it.
Another way to think of it: why label a button with the word “Back” when the button itself is already shaped like an arrow that is pointing back? If Apple wanted that button to be labeled “Back” then they would have designed it as a square button with rounded corners like the “Edit” button is.