Not always. Most sports titles, puzzlers, and MMOs are examples of games we regularly won't "Finish" before posting a review. Mostly because they might not have an end. As a general rule, the editors should play an assigned game as much as they need to in order to give a fair, in-depth review that can accurately represent the overall experience. This means that we might spend as little as 1-2 hours with a casual puzzle game or more than 100 hours on longer more "epic" titles. It really is on a case-by-case basis and is up to the individual editor.
That said, we always try to complete every game that is "beatable" and usually do so more often than not.