You can print it at home. or wait until you get to the airport, whichever is more convenient for you.
Basically, if you've checked in at T-24 to get your spot...when you log in subsequent to that (and before your flight obviously), it'll look like you are checking in again, but you get the number you locked into when you checked in on your mobile device.
I do that a lot...esp with early flights...I check in from my phone in bed instead of getting up and taking the time to fire up the computer. Then later in the day I print the boarding passes.