I am interested also. The best season should be from say may-oct with Sep-Oct the peak season as lots of visitors wanted to see jiuzhaigou with it's falling yellow leaves.
Make sure u avoid the Golden week from 1 Oct and maybe a week before and after. So the best is to go during Mid Sep or Mid Oct. Usually there will be photography tours organised by the chinese photo mags in their Aug-Oct issue... i think those should be good.
If you join a non-photog tour, i think u will missed lots of things and has no chance to wait for the right lighting, use tripod etc...