I was a Computer Science student and teaching assistant at the University of Toronto for a number of years (about a decade ago), and the 20% rule was in full effect: "On a test, if you write 'I don't know', you get 20% of the marks for that question"

As a student, this challenged you to know and understand what you know, and what you don't, and promoted the idea that knowing what you don't know is, in itself, valuable.

As a teaching assistant marking tests, this virtually eliminated long, tedious, often vacuous answers where the student was clearly throwing darts and hoping for a mark or two.