This speculation about taxes etc puzzles me. I am assuming the implication here is that there is a form of sales tax payable. In Europe, the equivalent is VAT (Value Added Tax) and if one pays VAT when purchasing an item and that item is then exported, the VAT is refundable so the purchase effectively is a zero tax item. Doesn't the US have a similar form of tax break available for exports?
Also, I thought I had read that Washington State gave Boeing all manner of tax breaks and, as for Qatari taxes, I am sure there won't be any as the airline is government owned.
Someone mentioned tax related wheezes such as flying offshore for the delivery paperwork to be completed. If that was the case, wouldn't all customers do the same? In the case of the Qatari aircraft, they could have been flown from Seattle to Victorville via a large loop over the Pacific? Also, if delivery had not been finalised when the aircraft left Boeing, why would Boeing allow the new owner's crew to make the flights? "When you own it, you can fly it".
There have been occasions when British Airways have taken delivery of new aircraft which needed new seating arrangements fitted. In those cases, BA flew the aircraft to London, then on to Cardiff for the seat installation. If there had been potential tax advantages to doing the same as Qatar, I am sure the BA bean counters would have spotted this and taken advantage.