It's generally more efficient to fly higher if the weight of the aircraft allows (ie there is an acceptable margin between overspeeding and stalling). However it is my understanding that some flight crews dislike going much higher than that in the daytime due to the higher UV exposure. ATC constraints also come into play of course if the airspace is congested. The 747-400 and 777-300ER are certainly capable of cruising at 40,000ft and higher if operational conditions allow.