You don't have to do it right at 24 hours, but if you want to print your boarding pass, or have it sent electronically to your phone, you will need to check in online. I did have an instance where they screwed up my EBCI and checked my DH and DD in at 24 hours, but didn't check me in. I waited until about 22 hours (thinking I was fine since I had EBCI) and I was B56. So I try to check in as close to 24 hours as I can now, even with EBCI. YMMV