Points from any given use year can be banked into the next use year buy not thereafter. Thus, for example, assume August use year and you get first allotment of points in 2008 (8/1/08). They must be used for trip that occurs between 8/1/08 and 7/31/09 unless they are banked into 09 use year, in which case they can be used for a trip from 8/1/09 to 7/31/10. However, those particular points cannot be banked into 2010 use year that begins 8/1/10. Nevertheless, points that you receive 8/1/09 as your 09 use year allotment can be banked into 2010 use year.
So when you ask about banking leftover points, it just depends on their classification at the time. If they are points that were previously banked, they cannot be banked again. If they are points that have never been banked, they can be.
This generally does not turn out to be much of an issue for most. Suppose you get 160 points Aug 08 and your first trip is Oct 09. Since Oct 09 is during Aug 09 use year, you need to bank the 08 points into 09 use year. You will have 160 banked points in that year and 160 regular that are issued Aug 09. If Oct 09 trip costs 160 points, you will have 160 left over. They use up banked points first for any trip, thus the only ones you would have left over are 09 use year points that you can bank into 2010 if desired.