That really depends on the taxing jurisdiction. In my state (WA), by law, for orders shipped or delivered to in-state addresses retailers must collect local sales tax based on the jurisdiction from which a product is shipped or delivered - "origin-based". This is nice and simple - I know my store's tax rate and I just have to apply it to in-state orders.

Starting 7/1/08, we will need to collect taxes based on the destination of the shipment or delivery - "destination-based"; this will be very disruptive to business because tax rates vary from county to county, city to city, block to block - there are even "regional" taxing authorities. The boundaries of these different taxing zones do not correspond to anything simple like ZIP code - you have to look up each individual destination address in a huge table to determine the exact rate for that address!

About half the states currently use destination-based shipping. I don't know of any case where the billing address is relevant to the tax calculation - tax is based on either where there item is "picked up" or where it is "delivered to".

And of course, other countries have their own rules. The best Ubercart can hope to do is make the architecture flexible enough to accommodate all these variations.