Sure, I would've replaced only two tires on one axle. IF i had a two- wheel drive car. But they do recommend changing all 4 tires on AWD. I have no background in car engineering, so, I can't access the importance of this. I can not access by how much the chances of my drivetrain going bad increase when I have tires in the back and in the front having a difference in diameter of 1/8 inch. And I know that rebuilding the drivetrain will cost me much more than the $30 for the shaving.
What I also know for sure is that I am going back to front-wheel drive when the Subie dies. No offense, great car, but the money spent on gas and having to worry about things like in the OP far outweighs having to drive in snow once in a while. BTW I found that the type of tires one istalls is at least as imporant as having AWD. I have Yokohama Avids, and with them the subie is just as bad (or good?) in the snow as my wife's Miata with snow tires.