Tax is often calculated per item. Especially in the days of the internet, some items are taxable and some aren't, depending on the item and your nexus. I would recommend calculating and storing tax with each item, to account for these subtle differences.
EDIT:
Not sure why this was downvoted, if you don't believe me, you can always check with Amazon: http://www.amazon.com/gp/help/customer/display.html/ref=hp_468512_calculated?nodeId=468512#calculated
I think they know what they're talking about.