If you have "proper" winters i would recommend using 225/40/18 at front and rear. The stretch difference isn't that big. Here is good comparison 225/45/17 at 8 and 8.5 wide
http://www.e90post.com/forums/showthread.php?t=85623
If you want i can take a picture of my 225/40/18 Continental IceContact at 18x8.5 wheel.