Changing the default settings in OmniGraffle 6 is straightforward – once you know the magic trick !!
The toolbar icons mainly have three states. Using the line tool as an example:
2: Selected (after clicking once) – in this mode the default tool settings will be used. Changes made via the inspector will apply only to the currently selected shape or line, etc.
3: Defaults mode (after clicking twice) – in this mode changes made in the inspector will become the new defaults.