Good Q! *thinking* Doesn't that apply more to range than to endurance though?
When looking at the endurance charts for the aircraft I fly (C172R&S), it says the chart allows for the fuel used for engine start, taxi, takeoff and climb, and the time during climb. It says nothing about descent. So maybe the chart would look different if it did...
It would take about 15 mins to descend from 12,000 ft at idle power, and about twice that long to climb up there. You'd use 5 USG for the climb, which is 1.2 USG more than you'd use in half an hour at 55% power at SL. If you were to shut down the engine at 12,000 ft and glide to landing, you'd save the 2 USG you'd have used for about 15 mins of SL cruise. So it seems you're right, it does appear to roughly add up!
