Go anytime between October to March if you like to wake up to some amazing aroma of coffee in the chilly mornings.
If you have feeling in mind... you can go pretty much anytime other than August / September when there is possibility of rain.
Best time to visit Coorg is from October to May. i have been there in May and still it was cold over there. rarely you will find rooms with AC as climate is chill. so you can imagine how the climate is. best wishes for your trip.