I agree with all of my fellow IAD'ers. You definitely want to avoid flights between 1600-1900. The airport is nothing short of neurotic. Early evening flights -- most are wide-bodies -- are leaving for Europe and the west coast. International flights from Europe either have just arrived or their passengers are finally clearing customs and making connections. Early morning flights from the west coast are arriving and people are connecting.

So, late morning/early afternoon or after 2100 is your best bet.