You will want to chop it when the trichomes on the buds (not sugar leaves) have reached the desired color ratio. General guideline is 10% amber, 80% cloudy, 10% clear but it will vary with personal preference. There is info on the different stages / colors of trichomes out there.
To answer the question, wet soil or dry soil, you can chop whenever. I personally like to have it a bit dry.