Hope for your flight to be delayed! Seriously, I've only checked in at IAD once; mostly I'm just flying through. I didn't have much of a problem when I checked in on a Sunday late-afternoon using the Premiere line (I was only a 2P then ... the horror!). You might ask one of the agents if they can get you a pass to use the quick security line. Sometimes at SEA they affix a "First Class" sticker to your boarding pass envelope which lets you get out of the cattle line. Just remember to dress nice and be curtious!