I have to second Nigel Osborn's suggestion, know what your RFM tells you. Adapt the data in the RFM to the variables present, especially wind. If you're flying into a wind, increase power from max range towards max power, and if you're downwind, decrease towards max endurance.
Personal rule of thumb, based on single engine, which usually have minimal RFM info, and single pilot, when I've had no free hands to calculate, experience: More altitude is a good thing, up to a limit, specifically, density altitude. Climbing to the DA that's the point at which one has to limit load gets the best range, more or less. If the climb to that DA decreases ground speed, then lower is better. GPS minimizes pilot stress so much!
When I've been minimal fuel on a cross country leg, which is always a headwind situation, it seemed getting the pig up to the DA at which I have to decrease load, and getting there as fast as possible worked best. If the wind was considerable- which again, always seemed to be true- then pulling max continuous power got me there with the most fuel remaining- at least accoding to available gaugeing, which in those aircraft is poor.
I don't use that RoT in multis that have real single-engine capability, but it's a good approximation in a old light twins, like the AS 355F1.