If you would like to default the Customer Ship-To Tax Code to a specific value, create a query and paste in the SQL at the bottom of the post. Change the the insert your default value where noted and create a formatted search, triggered off of the address name, on the State Tax Code field. The query will retain any values that already exist in the field.
SELECT
Case When $[$-178.12.0] = ”
Then ‘0’ /* Insert your default code within the single quotes */
Else $[$-178.12.0]
End As Code

 

Thanks to Ed Monk of SBONotes.com