I have seen both, the curtain moved forward and backwards, depending on how packed the GO cabin is. My tactics is to take the best available aisle seat (I am an aisle sitter) as close as possible to row 2 (I hate row 1, because of all the luggage policing, and that your luggage lands too far behind you typically - never understood why almost all airlines have this problem). Then at check-in I move to a better sit if available. At this time the move of the curtain typically has happened, and only rarely things change again. I do check the seat map once again when I arrive at the airport.

Despite this, it did happen to me that at the airport I have been moved into a middle seat, but then just before they close check-in sometimes seats open (cancellations and mis-connects, I guess). I have not been in a middle seat for a long time on SK now. But on LH, SN, and LO, that is a different story ...

Oliver will likely tell us that this is the theoretical seating in action (and he is likely right...)