Are there differences between test dates?

<p>I've seen a few test prep programs that have little blurbs saying things like "not all test dates are equal--find out why!" and "find out which test date is right for you!".</p>

<p>I already have a lot of test prep books, etc, but if there's differences between test dates that can give an edge, it might be worth it. Has anyone had any experience with either any testing companies that claim this, or heard about this supposed phenomenon and can vouch that it's worth the money to find out?</p>

<p>thanks!</p>