The other thing is that if you take the winds aloft forecasts, say FL120 v FL160, the error might be as much as the actual difference, whereas the jet stream is probably forecast pretty well.
Generally, with a tailwind, I will go higher especially if o2 is not an issue. One can quickly see (on the GPS) how the constantly recomputed ETA shapes up, so one can stop climb when there is nothing else to squeeze out.
In a steady frontal weather one can expect more wind higher up, but if flying in a large-scale high pressure zone, the wind could easily be all over the place, varying -20kt to +20kt seemingly randomly, several times during just one flight.