Lady
I've asked in ATC as you suggested
By all means ask but ATC don't make the strategic decision as to which route "we" take. In the hours running up to a flight the
airline will run multiple variations of flight plans through their flight planning computers, probably with varying lateral and vertical profiles, and ultimately pick the plan that works best (and as fa2fi says it's usually just down to cost)- that plan then gets "filed" with ATC. ATC can in effect veto certain parts of routes (e.g. due to forecast/actual route congestion, volcanic ash, etc, etc..) and/or they can insist we follow set routes in places (e.g. Organised Tracks on parts of the north Atlantic, inbound/outbound routes at airports), but they rarely (if ever) intervene and insist that we follow a specific route from departure to destination, certainly not on a long haul flight.
fa2fi has made a good point about "capped" levels and the like. Just as a +1 to those comments don't forget (?) that this is a three dimensional business and jet streams are three dimensional beasts. At any given point there might be a howling westerly head wind at 30,000 feet whereas it might be slack calm directly above the same point at 40,000 feet. So the "high flyer" going from A to B might quite happy and efficiently fly a direct line over a jet stream, whereas those also going from A to B not able to cruise at as a high a level for whatever reason, (e.g. weight and aircraft performance) may find it's more efficient to take a longer route around the jet.
There's really not any great mystery to this, just don't make your seat choice based on the previous year's/week's/day's flight plan...