You might be thinking of a "cruise climb", where you try and keep the aircraft at it's optimum level as the fuel burns off. In reality it's usually a series of 100 foot steps rather than a smooth continuous climb and in any event due traffic it's rarely if ever available over most parts of the world.
Standard procedure for the Concorde in supersonic flight, I believe. Of course the Concordes were pretty much alone on their supersonic cruise flight levels...