I just booked a Y award IAD-FRA-LHR, and returning LHR-IAD. Taxes were about 260.

If that's primarily due to ex-LHR, does it make any difference if I were to route the return LHR-FRA-IAD? In other words, does the ex-LHR tax depend upon the immediate destination (FRA) or the ultimate destination (IAD)?

As far as the German taxes go, my take is that they don't apply since FRA is only for connection and not a stopover.