I agree and disagree? A bigger rim with a shorter tires is going to do two things to the suspension. It will have heavier unsprung weight from the tire and rim and dampen vibrations from the road less. This means the suspension has to be stiffer (higher-rate springs, higher-rate shocks) to deal with the heavier tire/rim combo, and it has to do more travelling since the tires isn't absorbing as much of the small bumps.
If your new tire/rim combo is near the same weight as the stock ones, or within a few pounds, your shocks/springs will be fine IMHO. The LS has 16 and 17 inch rim options that all weighed different. This is really a pretty minor thing to look out for. A rim upgrade can be even better for the car if you go after the light-weight rims. Just make sure they are designed to be light-weight and not just el-cheapo rims that will bend in half the first time you hit a speed bump. Worse case scenario the suspension reacts slower and the car starts feeling looser. Chances are you might not even notice it really.
The tire itself actually acts as a cushion between your car and the road. The stiffer the sidewall and smaller the profile the more of the roads bumps will be transmitted to the rim and the rest of the car. That means the suspension has to move more and faster for the same stretch of road. This can wear out shocks, bushing, balljoints, etc. It will also increase NVH inside the car. I still wouldn't really worry too much about it though, so you only get 60K-miles out of a part instead of 70K-miles. It's all just a cost of doing business really.
As far as suspension geometry, bigger tires and different rims can change the alignment. Depending upon how it changes it (or how much) you might need to install Camber bolts to get things back in spec. Since there are a ton of people (including me) who have 18's on their LS without alignment issues, I think you are good. You should always get an alignment done with new tires anyways!
That's my opinion on it anyways.
