Is there any way that a customer can be set up as a Default Tax Exempt customer, eliminating the need to manually update the tax field as a 0% tax code when creating an invoice. For example if there was a Tax dropdown box where you can select Tax Exempt to override the tax setting on inventory or other default line item entries on invoice.
Customers have no tax codes associated with them.
There is. Every line item has a tax code box that can be edited to apply any tax code that has been enabled, regardless of what may have been set for an inventory item. But these are not associated with customers. They only apply to line items. The reason is that some customers may be exempt for some items (food, for example) but not for others.