I would be wary about doing this. If you get a better score you'll want to tell the schools you applied to about it, but they can easily see that you had checked the box that stated you weren't taking it again.
If you did worse and got in somewhere, schools could find out about your worse score and might think you're a shady fellow.
It'd be a different story if the MCAT wasn't done through aamc, but with all your stuff at one central location it doesn't seem to offer any real benefit.