In the 757/767, normally this is requested so to fly at the FMC's computed "Optimum" altitude. Factors that go into this computation are weight, winds, OAT, buffet boundary, etc. And most importantly, "cost index" as entered by the pilot into the FMC (numbers equate to high speed, Long range cruise, etc.).
Normally, as optimum increases as fuel burns (100 foot increments), that value is entered into the FMC on the cruise altitude (FL) line and entered into the MCP. The block altitude granted must be written down as there is no way to program the aircraft to fly optimum and/or block altitudes.