When your plant has reached its flowering point it is actually better to trim certain leaves.

Fan leaves which are blocking light to buds should be trimmed. Trimming these fan leaves will make your plant more potent. In response to the increased light/heat at the buds your plant will try and protect these buds by producing more THC. So timming fan leaves to get more light to buds is good.

Fan leaves which are blocking light to the general plant can also be trimmed but only do this when it is needed.