Of course in the real world, you'd attempt to climb or descend to take advantage of winds. But from the theroretical point of KC's question, and once again, speaking of only fuel burn (and not the other many variable costs), speed adjustments into a headwind gives diminishing returns in specific range very quickly.
Depending on your a/c, anything more than 1-2 kts increased indicated a/s per 20 kts. of headwind wil decrease your NAM.
kkbigjet-
I think he's talking about any given altitude. In that case, flying best specific range speed would cause you to decrease the optimum airspeed as you burn off fuel.
and... your speed adjustment into a headwind should be very minor.