Also, a clarification. The issues that are selected should be biographical for that character. They get married, divorced, change costumes, gain powers, etc etc. Not so much appearance related (except for first appearance, which is the superhero version of a birthday I guess)
EDIT: So in my spider-man example the points I highlighted (this isn't all the ones he needs, just the ones I could think up off the top of my head)
His first appearance & Origin Story
He gets his job at the Daily bugle
He graduates highschool
He meets Gwen Stacy
He Meets Mary Jane
Captain Stacy Dies
Death of Qween Stacy
I should perhaps add in his meeting of Dr. connors (who makes often appearances to "help" spider-man), his meeting of Harry osborn...
I added major events for Captain America (http://cmro.travis-starnes.com/character_timeline.php?char_id=119
) as further example.
I guess the goal of this is if you were to read these issues, you would have a good sense of the history of this character.