I would like some clarification on the differences between using zip5 or zip9 in SalesTaxII.
Using the zip9 (zip+4) is highly recommended when doing calculations with SalesTaxII. The '+4' allows for the correct county to be returned. That is what it is used for in SalesTaxII calculations.
Using this address in Nevada as an example:
11555 S. Virginia St.
Reno, NV 89511 9307
Using simply state and zip5:
Lavender County is returned.
Using the state and zip9:
Washoe County is returned. This is the correct country and as you can see, impacts the tax rate.
The USPS site allows you to find the zip+4 or there are many third party utilities available.