Our NEOs tend to like living a couple thousand feet below REC MAX, and even then the margins can be very tight at altitude when heavy. Some examples below from early on. The trop heights were correctly programmed into the FMS, and both were using low cost index and bracketing the optimum altitude within 1000 feet (per FCTM). First one is from a fairly heavy flight. Cruising at 350 with a REC MAX at 367. The second is from a lighter flight. Cruising at 370 with a REC MAX at 389. The first one was resolved by increasing Mach by 0.01; the second one by descending back 1000 feet.
In both cases smooth conditions with occasional light turbulence, but didn't take much air movement for the speed to decay. In these airplanes it seems like when the managed cruise speed gets within about 5 knots of Green Dot, the thrust margin to recover from a speed decay gets a lot more critical; this is when the A/THR gets "sloppy" as was stated previously.
Moral of the story in the NEO if your flight planning people offer you a high altitude right off the bat, if there is any degree of turbulence or higher than average temps at cruise, stay at least a couple thousand feet below the recommended maximum, optimum or computer-planned altitude be damned.
