From the I.B.I.S. Products Blog, Jack Sowers points out that in addition to supporting zip codes for tax jurisdiction identification, Tax Assist for Dynamics GP also supports and optional county code to fine tune the specific taxing authority. This comes in handing when zip codes span counties.
Tax Assist for GP Supports County Identifiers
Archived post from 2009. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.