If you live in a country with multiple tax rates or multiple currencies, you can enable support for that under Setup » Basic » Payments.
Check the "Custom tax rates" or "Multiple Currencies" box.
From now on when you edit a resource, plan, service or add a one time charge you can choose the tax rate and/or currency. Of course your invoices will reflect the updated tax rates, too!
All new tax rates or currencies will only apply to NEW invoices generated from the time the change takes place.
Let's move on to analytics.