To add to the eminent J_T's comments, most airliners use a "cost index" which balances fuel cost, crew cost and maintenance cost. Maintenance is usually accounted for by the hour, so added flying time might reduce burn, increase maintenance and, for that matter, crew costs. The index is a formula set up by the airline based on its particulars. Each sector will have a cost index for the crew to enter into the Flight Management Computer (FMC) and an optimal profile will be flown including climb, cruise and descent speeds, flight level (if ATC can accommodate) for that sector.
As John said, there are fairly limited range of levels and speeds that can be flown, basically FL 280 to FL 410, with levels above FL 370 only at the end of a long sector or a light weight take-off. Speeds are restricted by level, turbulence and range required, but typically for a B747 between M.82 and M.86, for the most part, a 744 pilot could chime in.
For a rule of thumb, each change of .01 Mach will shorten or lengthen the flight 1 minute for each 600 nautical miles flown.
GF