Fan leaves serve the purpose of gathering light and then turning that light into food. I have heard alot of folks like to trim their fan leaves down, but thats only after the plant has started flowering and is well into the budding process.
Most people only cut off leaves that are casting shadows on some 'important' part of the plant, like new side-shoots in the 'armpit' of a other leave. This can become interesting during LST or while cutting up your plant to generate more tops. I only cut up my plants during vegging, never during flowering, for its true the leaves make sugars which the plant converts into buds later. See what I mean in my first post of my first thread right here : http://forum.grasscity.com/showthread.php?t=91096 No leaves, no sugars, no buds. Simple. It gets better. Often, during flowering the buds suck the life out of the leaves because they cannot get enough of everything through the roots. So the leaves do not only generate power, but they also act as batteries. Remember this!