The best time to trim them is during harvest. To cut or not to cut the fan leaves is a running argument around here. The gist of it is that those big leaves are responsible for photosynthesis and act as the plant's "body fat" during the flowering period. The nutes stored in them are used by the plant to build buds. I was on the wrong side of the argument for a long time. Finally someone here told me to "stop trimming them and watch your yields double" I did and they were right, I was wrong. It had to happen once in my life...