Warning! If you are allergic to Cedar, or Japanese cedar, (or Monkeys, for that matter) bring asparin, antihistamines, or Gauze mask. I went there in April 2004. I loved the temples, and hated downtown, but I could not breathe due to an allergy. I picked up gauze mask at the subway station in Ginza, tokyo (people HATE it when you cough on the train), and that allowed me to breathe until we returned to Utsunomia.
If you are visiting Nikko in the Autumn (Fall), be prepared for crowds as I believe this is the busiest time of the year. Fortunately I was there late Spring.
Japan's rainy season lasts about 40 days during the months of June and July. Also, August to October is the typhoon season in Japan