Steve,
You do not need to shock unless you have measured combined chlorine or unless you are battling algae (or sometimes if your pool is cloudy, though other factors can cause this that chlorine won't fix). You can always shock after a heavy bather load or a heavy rain if you wish, but it isn't necessary. Also, the combined chlorine will get reduced somewhat slowly so even if you measure combined chlorine at one point in time (immediately after swimming, for example), it could go down later (assuming you have chlorine in your pool) especially after exposure to sunlight.
The pool store is wrong that you need to shock weekly. See this thread for a small sampling of how often users of this forum shock their pools. It seems to be about 0-4 times per season, with most only shocking once or twice per season.
Richard
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