Is it advisable to PhotoRead test preparation books, i.e., Princeton Review or Kaplan books for GMAT, GREs, etc.?

My concern is that PR would skew the results of the sample tests included in the books. The books have several sample tests along with answer keys and detailed explainations for the answers.

Hence the dilemma: if you PR the whole book, you are exposing your other than conscious mind to the answers, so whatever you score on the tests, it wouldn't be an accurate reflection of your true test taking abilities. It's like peaking at the answers ahead of time.

thanks,
Publius