There is a "sweet spot" for aircraft, and it varies depending on aircraft type and conditions. Generally, the higher the aircraft flies, the lower the air resistance and the more efficient it becomes (good speed for less burn)..this is impacted by payload as well. Too high, and the engines can no longer burn fuel and they "flame out". Too low, and they have to work harder, buring through fuel faster.

That said, reaching that altitude requires a climb. Sometimes a long one, which burns more fuel. Descent at idle at the end of the flight makes up for some of this higher burn at the start of the flight profile.

If there is a really good tailwind, crews will aim to take advantage of it as this provides a higher speed and provides some cost savings too as lower thrust is required. Bear in mind that TOO MUCH fuel savings could leave the plane heavy upon arrival, a situation crews want to avoid. If there is a strong headwind, this decreases speed and requires more thrust leading to increased burn.

Some of you may remember the incidents a few months back on some westbound TATL's that had to make unscheduled stops in Gander due to burning through fuel in headwinds.

Anyhow, strong enough head and tail winds can cause turbulence and if they are gusty, even more so. Airspeed can suddenly change by multiples of 10kts in sudden winds. The cost savings and fuel burn then gets inconsistent which is a headache for crews and uncomfortable for pax. You tend to have to reduce speeds in turbulence to reduce stress on the airframe..which leads to increased flight times and delays if it goes on long enough.

Crews are generally instructed to make pax as comfortable as possible and look for the stable air, but they are also instructed to stay within budget on burn and on schedule with departures and arrivals. There are consequences for generating a late and for burning through fuel when it may have been avoidable. Nobody wants to be the pilot that messes up logistics constantly. What is the lesser of two evils? Being late and exceeding budget or a few bumps and green faces? Which scenario do we think gets crews disciplined more?

If some old lady insisted on not sitting down prior to push back and they held the flight for a late full fare F etc etc and the crew has a chance to make up some of the lost time in a solid jet stream but may see a few more bumps in it, they will probably do it. Especially if their last few flights were similarly delayed. It is not soley the flight crews decision most of the time..they will contact both atc and the company first to request major changes in the flight plan.