The nice thing about planning VMO/MMO is you will have buckets of fuel as BOAC states. Remember that the additional fuel is to ensure you have the correct fuel at the crit point, and that fuel is based on your ETOPS divert speed.
On the down side you may have other systems that are more limiting than the fuel. For example a low rate fire suppresion system in your cargo compartement. It only needs to cover your diversion time. So if you planned to divert at MMO but choose to divert more slowly you could be inflight longer than your suppresion system lasts.