I have set out to find the origins of this strange theatre-custom. I have even employed the services of a reference librarian (we're related). We haven't come up with much, so much of the following rant is educated speculation. If you find better sources to the contrary, please elaborate. (Resume)
The OED defines curtain-call as "a call by an audience for an actor or actors to take a bow after the fall of the curtain." What this means, ladies and gentlemen, is that the AUDIENCE, not the actors, are supposed to lead curtain call. Anything else is self-indulgent, ego-massage.
Bowing is (again according to the OED) "a token of respect, reverence, submission." Until the beginning of the last century, actors were considered no better than criminals, so to play before ANYONE they had to show respect, reverence, and submission to their betters. In fact, actors would bow to the audience before every scene, in a sense asking permission to speak.
There is little acknowlegement of the humility of the custom in the self-congratulatory prancing I see more than I would care to remember. I am not suggesting that we should return to those days of gratiutous self-deprecation. To bow more than the audience asks you to is self-indulgant and rude - clapping hurts.