I think the style of the wheel is important when considering 18' or 19'. Some wheels only look good in a bigger fitment where as others suit various sizes. What wheels you looking at?
You will only get rubbing if the offset or width of the wheel is incorrect so, as a rule of the thumb the MKV platform is ideally suited for offsets between ET42 - ET50 and wheels widths between 7.5-8.5' whether lowered or not. If the wheel you are looking at is within this ranges you should not have any problems with rubbing as long as you don't lower it too much.
I have just recently changed from a 19' wheel to and 18' and the difference is substanial, the ride is a lot smoother and the car seems to handle better also