Because the GMAT is adaptive, you won't get a reliable idea of what you might score by doing timed sections. You'll know how many you're getting right and wrong, and that's a good start. I haven't actually seen anyone post this idea before. Rather clever, I must say, and better than just doing practice questions!
But I would take a practice test at least once a week during your prep so that you don't find yourself with one week left to get used to a CAT. There are plenty of good tests out there. Between the PR tests and GMATPrep, that's 6 right there. Find the older Powerprep tests and that's two more. Those three sets of the tests are the only ones I recommend; they're the most reliable for predicting your performance on test day.