Are you asking about tax exempt by location or by tax status regardless of location?
Abolabo is answering by location.
Here's my suggestions if you are asking about tax exempt customers - with a tax exemption certificate
1. You can use customer groups to manage this. When account is first registered they will go to main group, upon satisfactory completion of what ever you require for the tax exemption (usually a copy of the certificate) you can then assign them to Tax-Exempt or what ever you name your group.
See docs for info:
http://docs.abantecart.com/pages/customers/groups.html2. Other choice is to run multi-store - one for regular customers, one for tax-exempt
Option 1 is probably easier.
Lee