Different planes have different design cruise altitudes, given by the manufacturer. You can find these in their Airport Planning Manuals or in Jane's All the World's Aircraft. I know that the first iteration of these values is usually picked after designing the fuselage, and before designing the wings and other elements.
What is this decision based off? Is it that a larger frontal fuselage surface merits a higher design cruise altitude to reduce pressure drag? What considerations do you think are important to make at this stage in the design process?