it is advisable to fly to chengdu before proceeding to Jiuzhaigou?

Yes, it's ok for you to arrive at CD first. After arriving, you can take a rest or go to the Panda Breeding and Research Center to see cute panda babies. Next day, there are direct long-distance buses at Xinnanmen Bus Station to Jiuzhaigou and they all leave in the morning.
Enjoy your trip. :-)