The act of writing the cheat sheet is often enough to remember I find. It's yet another repetition of the material, just like doing labs and practice exams. And if you wrote the cheat sheet yourself, you also often know "where to look" for something specific, even if it's just to be sure you didn't remember something incorrectly and you really do only need to look at it for a few seconds.

So in my book (pun intended :P), allowing and actually encouraging a "cheat sheet" is a good thing. Open book is worse, as it's usually way too large and badly indexed. And who's gonna use an actual book in their actual job anyway?