I agree that it doesn’t hurt your chances to do online checkin, but in the case of a continuing reservation when the room assignment is so important, I haven’t noticed it saving me a lot of time when I am at the front desk and they have to double check everything anyway. Continuing reservations are the only time I don’t use online check in. For all other circumstances I do. The time savings probably depends a lot on the CM you end up working with at the desk.
The important part in my opinion is communicating at every chance you can that you want to stay in the same room if at all possible, especially when checking in for the first reservation.