It also depends on sidewall height for the tires. Low profile tires will not protect your rims as well as those tires with a bigger aspect ratio. If your running around a town known for it's potholes with 35 or 40 series tires, you're going to end up with bent rims, regardless of the size of the rim.
I think OEM tire size is 225/45 for the 18's. A 225/55 on the 16's would give you a little more protection without sacrificing too much performance. You could probably move up to a 55 series tire on the 18's without any concerns about driveability. Sidewall height for the OEM 225/45 is 101.25 mm (45% of tread width). For the 225/55, it increases to 123.75 mm, about 20% larger. The larger size will fill the wheel wells a bit more.