That's what I was thinking, climbing up there burns some fuel, and only so much can be recovered into speed through a slow descent which airliners typically don

That's what I was thinking, climbing up there burns some fuel, and only so much can be recovered into speed through a slow descent which airliners typically don't do, they hop between Flight Level. So I suspect only long flights will be operated that high, and there will be some calculation for the optimal altitude for shorter flights.