well, i usually trim periodically throughout the grow, i usually trim off all the lower growth before i stick them into flowering, Im also a great advocate of trimming dodgey looking leaves, you can cut a leaf in half and the remaining half a leaf will function just fine (& perk up a hell of a lot if what you trimmed off was unhealthy or yellowing etc). They say not to trim during the last few weeks of flowering, so heading that advice i usually take off the bigger (and usually by this point a little yellow) fan leaves to give more light exposure to the buds in week 5/6 of flowering. Dont know how much trimming really does for you but i certainly dont think it harms the plants.