If you already have selected seats, checking in online for a flight originating outside of the US does not seem be be very usefull. I tried to do this for my AMS-IAD flight last week, but could not print my boarding passes. At checkin in AMS the gate-agent told me online check in only works for selecting seats. She had to re-enter my pasport information as well as the address I would be staying in in the US, while I had already provided both at my only check-in. I was also only added to the upgrade waitlist for my connecting flight at that time, not during my checkin attempt online the day before.