I've been trying to get better about using the "Price Paid" and "Shipping Paid" fields recently. Once I started making a habit of it the paradigm shift with most online retailers now collecting sales tax occured.
It would be useful to have a separate field for "Taxes Paid". This would allow folks like me trying to track their spending better to do so more accurately while (theoretically) maintaining the integrity of the "Price Paid" field. The same toy here in NC at retail price costs a consumer 6 to 7.5% more than it would for our pals in NH, OR, etc so I didn't want to start lumping that in with "Price Paid" and skew those numbers.