And here's something I learned when we flew home on USAirways 2 weeks ago (MoBay to Charlotte):
When we arrived at the airport I realized I had screwed up and selected seats in different rows for my wife and myself (honestly, it was unintentional!).
I was able to select different seats on the self-check-in screen - wound up with BETTER seats that did not cost extra.
It may have helped that we were at the airport way early (Sunday morning - no traffic).