Alan,
That all depends on the excess engine power and the installed power vs the transmission rating.
For helos that are vastly over-powered, the extra engine power is a loss at low altitude, because the fuel burned is large when compared with an otherwise similar single engine helicopter. Overpowered helos get more efficient as the outside temperature rises (this makes the engines put out more thermodynamic power - operate at higher turbine temps) or as the altitude is increased (for the same reason.)
These helos like to get up to 7000 to 10000 feet, especially if they are at partial gross weight.
For more normal helos, 2000 to 4000 feet might be best, if at partial gross weight.
As Hmmingfrog says, any significant headwind wipes out this advantage, but if there is a tailwind, most helos favor a climb. Even in an S-76A (a basically sea level helo) I find that 7000 to 9000 feet is best for long range cruise if there is a 10 knot tailwind component. I can often fly almost 500 NM on a normal gas load if a 10 knot tailwind is available at 8000 feet.