We haven't noticed any difference in results between batches. The only difference we've seen is that summer batches get more applicants, presumably because graduations mean more people are free to do something new June 1 than January 1.
I had hoped one could perhaps see the effect of something like Subsyndromal Seasonal Affective Disorder (affects ±14% of population) or Seasonal Affective Disorder (http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002499/).