Sales order requires tax code error

Hello everybody, i need your advice if anyone has any ideas to handle this issue.

We have setup customer master by line of business, and our customers are setup with tax codes, so that when orders are entered the tax codes get defaulted in. Occasionally we do have orders that do not get taxed, so we try to remove the tax code from the sales order. But system seems to insist that I enter a tax code for the order.

Through trial and error, we discovered the only way to save the sales order is to first, remove the tax code from customer master, and then save the order, and then put back the tax code into customer master (so the next order will have tax code). This is very time consuming for us.