I think december is too late. If I am not wrong Dec 1 is date you can expect score; ets somewhere warns it might need around 2 weeks to forward scores to universities. So take a decision accordingly. Also visit the websites of each universities for admission schedules. Some top US universities I checked have deadline in mid-december. And I feel waiting till deadline might decreases chances of selection as shortlisting starts as soon as procedure begins in september. So, early bird gives first impression. And to overshadow him you can't be just equal to him but need to be better than him. So unless you are hardly pressed to postpone GRE, don't risk it.