I guess that makes sense, and I wasn't actually aware that T-45 check-in was a documented rule for domestic flights without checked bags (never had a situation where I wasn't able to check in earlier). So it seems that realistically, the cutoff is something like T-60, which gives enough time for the ticket to be issued for the T-45 check-in cutoff. However, I guess some agents will still make the effort, given that the computer's check-in cutoff is actually T-27.