While displacement creates a nice rendering effect, it's weakness is edge conditions. The object's normals affect the displacement. To create the hard edge of a cube, form•Z will split the normals along the edge (ie one pointing in each direction of the adjacent faces). This will also be reflected in the displacement as you have observed.
The workaround is to make the entire object render as smooth (Attributes->Smooth Shading->Facetted Objects->All Faces). See attached UI and resulting rendering. Note that there still can be slightly “roughness” where the patterns align unless you have a pattern that is designed for alignment (like the second example).
The displacement uses the object's texture controls. A technique for setting up the mapping is to make a material with the diffuse texture that uses the desired displacement map. Use the texture tools to set the mapping type and parameters. If you do not want to use this material for the result, then assign a different material to the object once the desired mapping is established.
We are consulting with Chaos Group if there is a way for us to have separate normal channels (and UV texture channels) for shading and displacement which would allow us to automate this behind the scene and give the desired shading across the faces. We will also be adjusting the UI to allow for a negative displacement amount. This might create better results in these cases.
We hope this helps!