Always calculate a tax rate
When setting up and VAT or GST based tax, I would advise that a tax rate is always calculated, so even when the tax is not known, use a tax rate call ‘tax not known’ for example.
This way, you will have a tax rate on every transaction, if one is not there then you know there has been an error rather than not knowing if the system has calculated the tax correctly or not! Secondly, if you don’t calculate a tax rate then you cannot report it. Having a tax rate on every transaction means that your tax reports will also have every transaction on them making the reporting thorough and reconciliation easier.
Of course there will be times when you won’t need a tax, on prepayments for example or if you create internal transactions that are just used for internal use and no tax is required, but if its for a sale or purchase, then have a tax rate.