I've been staying through the credits at movies since the age of thirteen. I know this because my 7th Grade French Immersion teacher lectured us on the topic one day in class. After watching some sort of French film, Mr. Sampson made us sit quietly until the credits had completely rolled. He told us that it was important to recognize everyone who put their time into making the film by sitting for a few extra minutes.
That really stuck with me and was fine when I was thirteen, or eighteen, or thirty. But now my time is at a premium and that extra seven or eight minutes at the end of a film could be better spent elsewhere.
But I'm also a bit of a completionist, and if I leave, I wonder whether or not I've missed some little gem of an easter egg at the end. I would have really kicked myself had I not seen Samuel L. Jackson show up after the credits rolled in Iron Man; eye-patch and all as Nick Fury.
Enter a great little iPhone app called Skip or Stay. It quickly tells me (based on user submitted votes) whether or not it's worth it to stay to the bitter end of a film. This is especially useful after drinking a washtub sized root-beer during Harry Potter and the Half-Blood Prince. Which you should just skip completely...it's not very good.