Theoretically, should there ever be a need to shock a pool if the chlorine is kept at the proper level?

When my father took care of the pool he would check the auto chlorinater daily (adding tabs). We would go an entire summer with out shocking the pool and it would remain crystal clear.

Now that I maintain the pool (and neglect to add tabs sometimes), it seems as though even if the tester shows up yellow (chlorine) algae can still grow. Did he just add more chlorine than me?

BTW, PH is fine, it almost seemed like the pool was easier to keep clear when it was low though.

Thanks,
John