In the case of the Corvette, much of the decision was for the aesthetic. The C5/6 both has kind of large[r] rears, especially in profile. The 17" front (on the C5) was enough to clear the brakes which is all they needed , then the 18" created a better front-to-rear balance.

I ran 18" at all four corner on my Z06 for the track (CCWs) and it definitely makes the rear wheel look smaller and out of balance.

The S has a really balanced rear profile relative to the front so matched sizes look really balanced. Personally I'd just stick with all 17 or 18" (in fact, I did, 18" SSR's )

 

I'd stick with one size.