There are questions to answer first... :smile:
Do you charge the same tax to everyone, regardless of where they're from? Or do you sell in a restricted area? Tax is dependant, almost always, on the address of the purchaser.
So, if the purchaser has not logged in, you have no way of knowing where they are, and therefore you can't calculate the tax. So to do what you want, you'd have to force everyone to log in before shopping. Is that okay for what you sell?
More info about your site, products, tax laws in your area, and so on are necessary before we can answer accurately. A link to your site is always a good idea.
Rob
Breakfast: the most important donut of the day.