If you are worried about some of these settings before running them, you can backup your preference files

All that the 'defaults' command does is alter values stored in plist files, which are located in /Library/Preferences (and for the user in ~/Library/Preferences).

You can add something like this to the top of the script:

    echo "Making a backup.."
    tar -pzcf ~/preferences-`date "+%Y%m%d-%H%M%S"`.tgz ~/Library/Preferences /Library/Preferences

You may need to sudo to read some of the machine plist files