NO, 737ng - you can fly any speed you are able. It is entirely down to the Captain.
If, however, you either run out of fuel going flat out or alternatively spend so long in the air that your cargo hold fire burns through the floor then you will look a little foolish. Have a look at my post #4 again?
It is purely for planning BUT that is the speed for which your planning programme calculated the CP fuel requirement. Remember also that is for the 'worst case' diversion - ie highest fuel requirement - which is depressurised.