This is a debate that will never be won or settled; however, like always I feel the urge to add my opinion.
Actors are paid to perform and only to perform and I could never be mad at an actor for not signing/posing for pictures - as long as it is handled correctly. I have heard horror stories about people (actors AND fans) being very rude. That being said, I think it is a very smart move to be as nice and accomodating to your fans as possible. This goes especially for those who are trying make their own music and need a large fan base to sell their albums. The people waiting at the stage door are the ones who will buy your music and go to the concerts your doing.
Its all about good PR. If you don't want to do it, you don't have to; however it *might* have negative impacts. The bottom line is that they have no obligation to do this, but in my opinion it is a smart thing to do.
I want to write music. I want to sit down right now at my piano and write a song that people will listen to and remember and do the same thing every morning...for the rest of my life. - Jonathan Larson. Tick, Tick...BOOM!