Cruise climb is without doubt the most fuel efficient cruise technique. Concorde used it (there were no other aircraft at 50,000ft+ and it was the only way to carry an economic payload), and Boeing considered it when they were developing the Sonic Cruiser which, had it been built, was designed to fly above the cruise altitudes of current commercial aircraft. But cruise climb can only be done when it is possible to block off bands of altitude, which in turn, is only possible when there are no other aircraft which could cause conflicts.
Current FMS equipment has what is called a Cruise Climb Mode but it is not a true cruise climb, it is only a means of climbing between the existing and a new cruise altitude. Climb thrust is used and speed is controlled by elevator.
An ideal cruise climb would need a new FMS mode, not difficult to do but not done because there is no demand for it from the operators since ATC procedures cannot support it without limiting airspace capacity by blocking off altitude bands. However, the cruise climb mode would need to command the optimum thrust (which would would change with pressure altitude and temperature), would control speed on elevator (the speed being varied to be ideal for the altitude, temperature and head/tailwind component - faster for headwind/slower for tailwind), and I guess one would then use the best range speed modified also for cost index. I am not a performance expert but I have talked with those in the know and it is not difficult to do.
Studies have been done on the possible benefits and I have seen estimates of between 5% for business jets (which I think is highly over optimistic) to less than 0.5%. Airbus recently did some work on A330 aircraft which showed that for a 4000nm flight between Europe and the Caribbean, at M0.82, at max payload, in still air, and with a continuous cruise climb to FL 390 the theoretical savings were 0.67% when compared with step climbs of 1000ft, and 1.2% when compared with step climbs of 2000ft.
Perhaps if ASAS Self Separation becomes a reality this could open the way for cruise climb techniques in low density oceanic airspace. In these days of expensive fuel and the need to reduce CO2 it would be worth trying.