As with many Government agencies, ours has a rigid structure around the use of fonts and formats that is based on corporately set styleguides. One of our key challenges in producing a viz then becomes how to apply these fonts to each and every worksheet that we produce which can result in several hours lost work in simply re-adjusting tableau default fonts to Arial 10 for example.
The ability to permanently change the program settings for the default font and formatting are not currently a feature of Tableau Desktop. There are a number of known workarounds, however there is an existing enhancement request that would benefit from your vote. Tableau uses its forums to gauge community demand, so your input will definitely assist. This suggestion came from the Tableau technical support team themselves.