If going to a smaller diameter tire, I don't think it would cause extra wear or anything like that, however, it will increase fuel consumption if you do mostly highway driving. It may help a little with city driving. Basically you are doing the same as changing to a higher numerically gear (i.e. from 3.42 to 3.73 gears).
But, the size you are proposing is actually LARGER than stock. The stock size is 245/65-17, which is 29.5" and the 255/60-18 is 30". So going to a larger tire will decrease RPM. When going 60 MPH on the speedo, you will actually be going 61 MPH.
Reference:
Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.
tiresize.com
If you want to go to an 18" without changing effective gearing and speedo reading, you could go to a 245/60-18 which is almost the same size (29.6") as the original tire. That is also the size on GMT360's that came with 18" tires.