A lot of comment in this forum that speeds below M0.83 make little or no difference to the total fuel burn. That's not surprising.
If you're at or close to Optimum Level in a B777, go to the INIT REF Page on the FMC, and insert CI=0 (Maximum Range Cruise). Inevitable, the scheduled Mach Number is in the range of M0.825 to M0.827, just short of the M0.83 mentioned. With Max Range Cruise JUST below M0.83, it's little surprise that there's little significant difference. You might note a slight gain in fuel on a very long sector.
Of course, if your cruise level is somewhat "Off-Optimum", you won't see these results.
The bottom line is that Climb and descent at CI speed will make a SIGNIFICANT difference to overall fuel burn, not advisable to fiddle with these. On the other hand, "fiddling" with speed during cruise, will, generally, have only small impact on overall fuel burn. Of particular value during climb is deleting the low level 250 KIAS speed limit as soon as possible. Deleting the speed limit at about 5000 feet will give you an immediate 400 to 500 Kg fuel "bonus".
Best Regards,
Old Smokey