Parson's Style Bookshelf - Screw Sizes

I have begun to build the Parson's Style Bookshelf   http://ana-white.com/2010/10/p.....f.html    and am curious about a particular screw size in the plans. 

First things first  –   I am using a Kreg Kreg Jig for the entire project.  I have the K3 Master System.

According to the plan, when joining the shelves to the "ends", it says to use 3" screws.  Considering that I am using the Kreg Jig, aren't 3" screws overkill.  I wonder if anyone has built this bookshelf using shorter (perhaps 2" screws)??? 

Thanks in advance for your help/suggestions.