This is just my personal experience, but I had 40 series tires on my car and I ended up bending 3 out of 4 rims in a span of 3 months...I was always cautious of hitting bumps/potholes, and I don't even remember hitting anything that would have bent the rims. When I had an alignment done, the mechanic put my wheels on the spinner and showed me that 3/4 of my wheels were slightly bent. With 18's you will probably have to go with 35 series tires which are even skinier

I guess it all depends where you live...Kansas City (where I live) has the 2nd worst roads in the U.S. If the roads are smooth where you are then go for it.