Trying to work out what kind of environmental details should be modeled as 3D elements or be part of flat 2D textures.
And I've come up with the Grab Test.
"Can you grab it?"
If something is protruding enough from an object that you could grab it, it gets modeled as part of the mesh. If it's too flat to grab, it will be part of the texture.
I think that will work well for the fidelity I want to work at.