Depends on what you want to do. Are you running all-seasons on these, or thinking of getting a dedicated set for winter? I have a dedicated set of 17s and boy, have they saved my bacon a few times.
I run 20s when it's not winter and so far I've bent a rim real bad on 190 and ruined a tire on 495. Blah.
Regardless, 18s will be FINE, but it never hurts to have extra padding. If you get the right 17s, this car still looks great on them, but obviously 18s will fill the gap a bit better. Here's mine with the 17s vs with the 20s: