It usually doesn't matter as the load controllers will know quite ahead of schedule (T-72 to 24) whether a flight is going to be overbooked and will start to flag people they want to move around. It is always assumed that Platinums and Golds will turn up, regardless of whether they have checked in or not, and they are last on the list for offloads. Sometimes if you want to be flexible and get a free return ticket, you can volunteer if the check-in staff/managers look especially stressed by numbers! I have just returned from a trip with quite a few op-ups and I never checked in online. On one sector I checked in 90 minutes before departure at a desk and still received it (at the gate).