i do not know when weignt becomes a consideration in current tatl planes. however, fully loaded may make them heavy enough to fly a few thousand feet lower.

on my recent trip back from geneva to iad, the plane flew south of england and ireland, not the great circle route. we had a 10-20knot tail wind the whole way. never gone that way before. had a bit of turbulance here and there. 10 knot tail wind at 33,000 feet instead of a 120 knot headwind at 36000 is a real cost saving.

someone commented about turbulance, and related them to head winds. don't think that correct. horizontal wind does not make the plane bounce.