Lots of interesting talk about speeds, but not much about maintenance cost. If you know (even fairly roughly) what your plane costs in terms of maintenance per time unit, then you can properly make up the balance of fuel cost and maintenance cost.
In a previous company we had some guys always wanting to go as close to service ceiling as possible, because fuel flow would be lower etc. Meanwhile, they were running the engines fairly hot and kept high RPM to get there and anything they saved in fuel was later basically burned up twice by the engine needing heavy maintenance much sooner due to running it hard all the time.