It's usually not true, and it undermines your confidence in the ability to do what you've already done at least ten or twenty or forty times. That is, to take a practice or real LSAT and score consistently.
I, for one, only dropped one point from my summer practice average, and I felt no different taking the real LSAT than I did taking any of my PrepTests (sans my diagnostic). Yes, You can disagree with me as vehemently as you want, but I believe that your raw score will be virtually the same as your PrepTest scores if you buttoned down all the hatches.
The problem is that people lift-off from the airport before they secure the cock-pit. Anyone who's been in an LSAT panic situation will understand my metaphor: suddenly, you may find your plane breaking apart under extreme duress.
Just thought I would make a public service announcement . . . good luck to all the Dec. test-takers!