I've cut it close before without checking in the day before. Not only did I have to stop and spend precious time to check-in, within a certain time period from departure the self service kiosks won't check-in and print a boarding pass for you. Then I had to wait for a ticket agent...

That was a bad morning. Somehow I still made that flight.

You never know what might happen. It's one less thing to worry about and line to wait if you have access to check-in.