To choose the default settings for Reports, you must first navigate to the System Preferences screen.

 From there, click on 'Reporter' in the User Preferences list on the left. Next, choose the default margin settings for all of your future reports. You may also select a default font and choose whether to display the standard header. 

Click 'Done', in the top right, when finished. When you create a New Report, it will default with your selected settings.