After each Rate & Boundary Import. The import process deletes Tax Schedules that are no longer valid, but does not remove those schedules from the address records. The Assign Tax Schedules routine updates address records with the proper tax schedule IDs. When a state creates a new tax or drops an old tax, any customer addresses that are subject to the effected tax jurisdiction(s) must also be updated. These are the kind of reasons why you must run the Assign Tax Schedules routine after every import.
If you are using the expiration date setting in the Allow Overrides by State field on the DZ Customer Supplement window. This process can update the customer's current exempt tax schedule ID with a taxable tax schedule ID if their exemption certificate has expired.
If you are importing customers into Microsoft Dynamics GP then you could run this routine after each customer import to ensure that they have the proper tax schedule IDs.