The max altitude in the FMC is the lower of the following
1) Max certified ceiling
2) 1.3 g altitude (JAA is .3g, believe FAA allows a little less margin of .2g)
3) Thrust limited altitude
Can't speak for the 777, but a 737-800 with winglets would be limited by thrust when ISA goes above approx +18C. Below that it will be limited by maneuvering margin, or max certified ceiling if at light weight.
My opinion is that you only climb close to max alt if you know it will be smooth at the higher level. However, if it's not smooth you could always descend back down to increase your margins again.
The .3 g margin is there to keep you sufficienly away from the coffin corner for obvious reasons.
Being thrust limited is not necessarily any better than being maneuver margin limited since the aircraft may end up stalling. If in HDG SEL on the 737NG you can in fact stall the aircraft during a turn since the thrust required may exceed thrust available.