You need to charge tax according with your local tax laws. Usually this means that if you are selling products from California, for example, you would charge tax to customers in California.
I would suggest contacting your local Chamber of Commerce to find out who you need to charge tax, and at what rates.
Remember that you cannot charge tax by County, only by State.
Also keep in mind that the address that the shopping cart uses to calculate the tax is the Shipping Address.