In my experience if you ask regarding 1-2 dances, everybody is helpful. More than that and you're getting closer to cribbing a program and the risk of offending rises with some callers. Timing of the request is also important. I do my best to not disturb the talent at any point their attention is required for the event's success and to minimize the time consumed, unless they seem to want to talk.

Typically I'll jot down the choreo from observation and limit my request to any bits I may be unclear on, like the name or author. In cases where I was dancing and unable to recall the sequence from memory, I may either request to take a photo of their card at a clear break or send an email request so they can deal with it at their convenience after the event.