With the on-line booking tool you can click on the "taxes" line in the fare summary and bring up a complete listing of the relevant taxes and surcharges. This is a tremendous improvement over the past.

I've tried a couple of dummy DONE4 bookings online using the same overall itinerary but with different start points (e.g. ICN-NRT-JFK-LHR-JNB-HKG-ICN and JFK-LHR-JNB-HKG-ICN-NRT-JFK) and have found there to be some variance in overall taxes and fees (+/- 20%) even after currency adjustments. In other words, trips originating with ICN-NRT and ticketed by QF on behalf of JL, have taxes up to 20% different from those starting with JFK-LHR and issued by AA. Note this is 20% variance in the amount of taxes, not 20% of the base fare, so e.g. one itinerary after currency adjustments comes in at, say, USD500 for taxes and fees, while another comes in at USD400. I think some synchronizing by OW is going to be needed in this regard.