I wouldn't bother with online check-in, either, if I were checking in at midnight. It would save you a minute or two at best.
The purpose of online check-in is to shorten the time you spend standing at the check-in desk. Your paperwork and keys are prepared ahead of time, and you get to stand in a separate line. But at midnight, I'd be surprised if there's any line at all at the front desk.
In practice, here's what the difference will be:
Online check-in: You walk up to the desk, show ID, the CM will give you your packet and keys and send you on your way.
Regular check-in: You walk up to the desk, show ID, the CM prints up your paperwork and keys and sends you on your way.
Either way, it won't affect your requests at all. If you don't get your king room, ask the CM checking you in if they can see if there's one available.