Currently there is no way to make customer tax exempt in nopCommerce project.
Yes I know that can be done manually by assigning a tax exempt role to that customer but that's a manual process. If a customer is placing an order, he/she don't wanna wait for the store owner to make him/her tax exempt. This is a very long process which is not feasible at all.
In U.S there are many online stores that cannot charge tax on the products from a customer because of state contracts etc.
It is a very important feature for many store owners / ecommerce businesses and it would be great to have this feature in upcoming future versions.
The best way is to add a field on the registration page ("Is tax exempt" Yes / No) - The customer will select according his/her requirements. Eventually the order is coming to the store owner where he can find out if customer is actually tax exempt or not.
If this feature will be configurable from the admin section then it would meet the requirements of all kind of store owners.