ISA deviation can make a huge difference especially if there is a large height band at high deviation. It made sense to fly lower and faster out of the band. Logging the temperature every 5000 ft soon gives you a clue to the best altitude for temperature to compare with actual winds. This worked well in B757 and B767 in Europe, I often found high 20000 to low 30000's would be better than optimums around 37000.