In my experience they have always been available even on sold out flights. They do fill up though, as Knope said, the best bet is to online check-in as soon as possible.
Originally Posted by
knope2001
Stretch is only available in advance to Summit members (not Ascent)
They aren't available to Ascent members in advance? Do you just choose the stretch seating at check-in and not get charged? Just made Ascent status (though still not showing it due to a bug with their system.)