When I lay out flight plans for the widebodies we schedule in the office I work in, we use different settings of seasonal winds - i.e. 85%, or 75%, depending on the mission. This is mostly just to get a ballpark figure on fuel burn and operating costs. I assume just like intra-US IFR routing, trans-atlantic and trans-pacific routing has preferred routing and travel lanes for crossing the oceans and polar routes that actually must be filed for on the flight plan.
But once you are in the air the game changes significantly. A lot of times pilots' can request deviations or new flight routing in order to hop in the jetstream or ask for different altitudes to avoid turbulence or lessen a headwind. This is especially true for east-bound trans-continental flights in the US. I've read and heard plenty of stories of planes getting in the 700MPH ground speed realm because of a serious tailwind.
Its like the flat escalators underground at ATL
- you may have to walk in the wide lanes between the escalators if the faster escalator lanes are too crowded, but if there is room and clearance, you can hop on, exert the same amount of work, but reduce travel time and overall energy used.
Unless you are about to sit on the ATL
flight for an endless amount of time, in which case I walk the mile