At 10 days out.
I am all for you calling to make requests, but also....
I used online check-in for an August SSR trip, and there were many choices on the check-in form, including location, pools, and floor level. I was able to get the area we wanted, and the highest floor.
There is a great thread on DVC resorts SSR with excellent info -- and sounds like you may already know where you want to stay. I think online check-in will help.
(This has not been my experience with other resorts and online check-in, where there are few if any choices on the form, but for SSR, perfect.)