You know, I don't really know! I'm guessing maybe you're getting the "last chance workout" language from The Biggest Loser? They seem to really believe in it. But I've heard that your muscles can actually retain quite a bit of water following exercise as a way to repair themselves. Plus, like you said, you wouldn't want to be tempted not to drink water while you were working out just to see a difference in the scale.
Now, that all being said, if you exercised every week before your weigh-in for about the same amount of time and took in about the same amount of water, you'd probably be making close to the same impact each time.