I prefer the evening flights because I have a really hard time getting up early enough to get to IAD for a morning International flight, especially as traffic is very unpredictable and the UA terminal in IAD is a nightmare early in the morning. I find it much more orderly and manageable to check in for an evening flight, and while I do need to leave plenty of time to get there, I don't mind as much. For an evening flight I can grab a bite to eat or a beer to pass the time. For the morning flights there is not much open yet and if I luck out and miss traffic, I end up with three very long hours to kill and nothing to entertain me.