First off - the documentation on the SFDC site is incorrect on this issue (http://goo.gl/ycjLs). The question keeps getting asked, people get referred to this link, and then they soon write back saying that it DOES NOT WORK.
In short, since the Amount field on Opps only calculates Sales Price, Quantity and Discount, the taxes (or any custom $ field) do not get added. Some of our products are fee -and are not taxed.
The method of using a WFR to update the Sales Price (as in Total Tax Amount__c) so as to update the Total Sales, to in turn update the Opp Amount figure does not in fact update the Sales Price.
Result: Our quotes have the wrong Amount.