Well, looking wrong is one thing and is subject to personal preference.
The big drawbacks for 18" wheels are, 1) significantly increased weight = higher inertia = slower car, 2) unless you can source 18x7.5" wheels, it's likely that 18x8" wheels and tires will scrape the fenders (there are ways out, however) , 3) if the offsets are off, handling and steering feel usually change for the worse.
I have 17x7.5" on mine with offset made for the GTV, not 156/147. The steering response is a bit dull and there's more kickback than 16". I'm using them because I love the look of the wheels, though I'm planning to return to 16" teledials in about a year or two.
But it is up to you, if appearance is your priority, by all means, 18" is
I know a few 156s here with 18" Sienas. They look
, but drives
compared to 16" or AR's 17x7 wheels.