That does not always work - have selected seats at time of booking only to find they are not the ones we end up getting when checking in. So always carry the confirmed seat allocation email as well.
Now if United would only stop moving me from the 11am flight to a 1pm flight when flying Appleton to O'hare it would be appreciated (3 years running now) and put the 767 back on the LHR / ORD route rather than 747 as well.