Of all the great works in the Beethoven canon, there is never an inappropriate time to perform his Symphony No. 9, “Choral.” For this reason, it is an auspicious beginning to the National Philharmonic’s 2011-2012 Season. Performing in its home at the Music Center at Strathmore this weekend, the orchestra, as well as the National Philharmonic Chorale, will be led at the podium by JoAnn Falletta, music director of the Buffalo Philharmonic, substituting for Piotr Gajewski.