Hi Mel,
the gradebook settings are handled by Moodle central code, and as far as I know there is no easy way to define a default value. At some point, I hope to have a look into it an see what is possible.
FYI, I would also like to set a default path in the file picker so that when your adding a new TaskChain the file picker will start in the last directory you were looking in. However, that too is handled by Moodle core, and I don't know of anyway to pass it the starting directory path, or have it remember the path you last used.
regards
Gordon