This is how we handle customers with different taxes or with tax and without tax

1. Create a customer category, WITH TAX
2. Create a customer category, WITHOUT TAX
3. Create a tax category, STANDARD TAX
4. Create a tax, eg. TAX 5%, select STANDARD TAX in tax category, select WITH TAX in customer category, set tax rate, eg. 5%, leave parent tax and cascade blank
5. Create a tax, eg. TAX 0%, select STANDARD TAX in tax category, select WITHOUT TAX in customer category, set tax rate, eg. 0%, leave parent tax and cascade blank
6. Create 2 customers with category, WITH TAX and WITHOUT TAX
7. For products, choose STANDARD TAX for tax category