I have done the low road and high road method....usually the high road was for cooler OAT or to get above bad vis.
For long transits....Wind was also an issue as with a tailwind it took less total fuel to meet fuel requirements thus more payload for passengers and baggage.
Some routes were pushing 250nm one way plus having IFR fuel reserves for an on-shore diversion.
Otherwise....500-1500 feet all worked fine.
I once made a ferry flight from Deadhorse, Alaska to Lafayette, Louisiana and never got over 300 feet for the majority of the flight.
My basic rule was fly high enough to have a decent forced landing site within autorotation range....if at all possible.
If there was no where to land....then it really did not matter so long as you had time to get a radio call off.