I generally defoliate right before I flip to flower to clean the plant up and ensure good airflow. I also strip the bottom 1/3rd of the plant of all growth as during flower that growth won't really develop very well. I defoliate again at day 21 of flower again making sure there's plenty of airflow and making sure no bud sites are covered. Any growth that looks really small and underdeveloped I will strip off the bottom of the plant for the same reason as before. After this last defoliation, you should wind up with a nice canopy that has all the bud sites exposed and a lot of space for air movement between the buds to help prevent mold and other moisture related issues.