No reason to not try and do it online a few days in advance. If it works - save a trip to immigration. If it doesn't, go there on or around the due date. That's what I do. Win-draw scenario!
In approx 8-10 consecutive online report-in's I've never been rejected. Not saying it (rejection) doesn't happen - happened to a pal of mine the other day. Just saying; nothing ventured, nothing gained.