I would exhaust all the kaplan practice tests, then aamc test. AAMC test usually give a great ball park, especially the last three. I mean you can def take ones that are being administered now but you won't get a score, so there is really no way to know how you did.
I WAS planning on doing the same as you are thinking but decided against it. for the following reasons: 1) You really can't predict how you did on the MCAT without getting your score back. 2) For this cycle of MCAT (current version, not the 2015 MCAT) you can only take the MCAT up to 3 times. The void MCATs count as takes. So you would only have 2 tries (if needed) to get the score you want.
There's only one kind of student who should do this - someone who has a diagnosed anxiety disorder for which they are currently in treatment and for whom the process of checking in at the testing suite will likely trigger an anxiety attack if it's totally new for them.
Mind you, I'm not talking normal nervousness or test day jitters. If you suffer from weapons-grade test anxiety to the point where you couldn't even function (work the mouse, write with a pen on the scratch paper) then it can maybe make sense to intentionally go in and plan to void the score as a form of exposure therapy or flooding.
For the other 99.99% of test takers this would be a waste of time and money and potentially risky. As Fedxup rightly notes, you've got 3 chances with the current MCAT. You only want to do it once, but you also want a back up "just in case". And to make sure nothing disastrous happens, you may want that third crack at it this year. To intentionally go in using up one of your chances is not a good idea. (for example, I've had hundreds of students over the years tell me a variation on this story: "the first time I took the test I was just flat-out unprepared. i didn't really study during my Kaplan class so I tanked it. Then I went to take it a second time but [life disaster] happened the day before/that morning. Now I'm on my third try.")
You're asking about "for extra practice" but there's dozens of practice MCATs out there that you can take, so no need for a real Test Day just for practice.