I am a complete newbie to the pool business. First year with a pool. I have only shocked 3 times this summer. During heavy rain events I have noticed that my pool liner will be slightly less "squeaky clean" than it normally stays. I have never had any combined chlorine reading above .5 but my intuition tells me that when something is going on in the pool like this I should shock to make sure. It only cost about $10 and the piece of mind is worth it to me. Now this goes against what some people believe here and with more experience I might change my protocol. It just gives me piece of mind. I also have a continuous heavy swimmer load and lots of debris gets in my pool during thunderstorms. FWIW