They'd have to play the matches since, at least as of the league tables as of this morning, these matches are still meaningful to the Nationals in terms of post-season seeding (vis-a-vis the Reds, as you state).
Chances of rain, per weather.com just now:
Today - 70% [it's raining outside my office window right now]
Tomorrow - 60% [this has increased over the past couple of days - rules out doubleheader if rained out tonight?]
Thursday - 20%
Friday - nil
So, worst case scenario, Nats are rained out today and tomorrow, have to play a DH on Thursday, end up as the #2 seed and have to fly cross country and open their series Saturday in SF. I guess we want Cincy to lose today and tomorrow so the seeding is wrapped up and these games don't need to be made up.