If you buy and the seatmap appears full (or you just get stuck with something like a middle seat), it's definitely worth checking every now and again as people move around. I got stuck with booking two transatlantic flights in non-aisle seats in the center, but I've been checking about once a day and have been able to move both to first to center-aisles and then to window seats (my preference).

I find myself wishing for the option of telling the system "I'll take this seat for now, but as soon as a window opens up, automatically move me," or something like that.