Well, I've done it many times before (as well as countless other people) and in my experience you will get better results, and much more consistent results by using a portion of the cake. Also, the "braun hefe" or brown, dried yeast on the sides of your carboy is something you are really better cleaning off. It has bitter, harsh hop compounds and dead yeast you really don't want in your beer.
I'm not a big fan of pitching directly on a yeast cake, especially not a low gravity beer. You are better off, IME, going with a portion of the slurry and a clean fermenter.