If you're Plat or Gold Elite, or if you're on a fare booked in Y or B class, you can request exit rows prior to your day of travel through reservations. The system should even allow you to choose an exit row on nwa.com. When you check in, therefore, your exit seat would be issued to you.

I've not heard of the 24hrs plus 15 minute rule for exit seats at nwa.com, though.