We just used United last week and checked in at the 24 hour mark. Our seats were not the same seats that we chose 6 months ago. They had my 8 year old DD in row 34 and I was in row 11. It was just the 2 of us. We usually fly Jet Blue so this was new to us. After talking with some other people I was told this is common for United.
This happened both on the flight down to MCO as well as on the flight back. We had to make the seat changes at the gate. The airline had to find someone to switch seats.
I was not happy about the situation since I booked seats with seat assignments 6 months ago. But, it ended up working out in the end.
So my point is, with United, no need to check in at the 24 hour mark.