I wouldn't call this a rule -more of a recommendation.

Its not recommended to leave lipo sitting at full charge - this makes internal resistance go up and lipos treated this way have been known to puff without apparent reason. I used to do it that way, but not anymore. I measure the voltage after running and if its not around 3.7v/cell, I put the lipo on my charger and charge them to storage mode at 3.8 - 3.85v/cell.

Then when I get ready to go run and need on, it is fully charged right before using. If you plan on running, chage one up and then don't go, its best for longevity to discharge them tp storage rather than leave the lipo sit for days or weeks at full charge.