Stick with your 17"s and I think you'll be happy to have some rubber around the rim when you whack a pothole or something in the road. I think 17" are a more practical size and IMO fit quite well but 18"s still do look good.
'12 Focus Red Candy SE Sport 5 spd
'99 Toyota 4Runner Sport
'11 Subaru WRX STI Hatch