Hi,
It is certainly not necessary to take a prep class and I have found many of the commercial classes to be underwhelming. Indeed, the entire concept of test preparation is usually a failure since most students need a higher standing on the measured construct rather than a series of hastily cobbled together "tips and tricks." If one has a knowledge of psychometrics many of the claims made by test prep companies range from the merely comical to the borderline fraudulent. However, some students still find these types of classes useful.
Hjort