Boeing recommend a flight level no greater than 2000 ft above FMC indicated optimum level.
Bearing in mind that optimum level will also change depending on cost index, speed and a/c weight, it does seem to be a much more consistent method of flight level decision making and will avoid the dangers of reduced margins.
Also a reminder that FMC recommended level is only valid if winds are accurately entered. It is not a figure to be followed blindly.