I don't mind when people ask if other people are coming out, though I understand how it may come off to some people. The actor may or may not assume the stagedoor fans are friends with the other actor they are asking for. And assumedly, people in casts should be friends and respect one another, and respect that different people have different fans.
I've always been the type who doesn't see signing at a stagedoor as an actor's "responsibility" or, "part of their job." It's not the right of the audience member to get an autograph, nor is the stage door an "institution."
This all being said, the absolute worst thing that I find unconscionable and inappropriate, is when people compliment actors on their prior roles in other works, rather than the performance at hand. It's classless and irrelevant. Like the Donna Murphy example above. I think it's the worst offense you can make at a stagedoor. Worse than following an actor home, or asking for a photo or a kiss, or asking " is someone else is coming out" It's just wrong.
Then again, to clarify, I don't see a problem with people complimenting actors by saying, "I saw you in ... rare show 10 years ago... and I decided to see you in this, and you were great" But with ensemble members, it's sometimes complimentary to say " I saw you in ..." which might be something they're not often recognized for. Just as long as people know the etiquette is to compliment the current performance, and not say, "You were great in ... previous show."
Stephen: "Could you grab me a coffee?"
Me: "Would you like that with all the colors of the wind?"