All that theory is great, but yes, you will save fuel in most jets if you slow down.
Old rule-of-thumb for the 747 if you get held down below your desired cruise level -> Keep the TAS to 500kts or 320KIAS, whichever is the lesser.
If you are stuck at say 28,000 instead of being at say 35,000', then you will probably have to slow down to about 0.82 - 0.83 odd, but you'll arrive (disregarding differences in wind component) with only a tonne or two less, over a seven hour or so flight.
No big deal.