If you have a customer that is regularly placing tax exempt orders, you can mark them as tax exempt.
Look up the customer, and "view" the customer details (or click on the customer box at the top of the ticket).
Expand the "More Info" section in the bottom right of your screen.
Turn on the "Non Taxable" toggle and enter the Non-taxable ID.
If you have an order in progress, use the blue arrow to make the current order non-taxable.
Next time you search for this customer, you'll be prompted to make the order tax exempt: