I would say anytime you can go. :)
We really prefer either September, October or January, February. The crowds are really down then with all the kids in school and the flights are cheaper then so you should be able to score a deal no matter where your flying from. January
And February are nice is you come from a cold climate and you should see whales then. It is more crowded then compared to September, October.
September, October are warmer and the ocean is a little warmer and calmer for better snorkeling. You should save a little more on your rental car rate during this time of year also.
All in all there is no bad time to go to Kauai!!!