If I were you, I'd note the potential exit rows and then check right when online checkin becomes available (it seems configuration 4 is the one that's different for economy class). The SQ seatmap will, usefully, tell you the seats that correspond to the exit row as well as those that do not have window views.

Definitely, try to check in as close to the 48-hour mark as you can. You can already ask your TA or SQ reservations to note your preferred seats. But don't get your hopes up too much, as other people are trying to get the exit rows as well, including premium passengers such as PPS members and those who are KrisFlyer Gold.