I know of one member who had 18s on his white teg and it looked pretty descent.
It's only a matter of how you set the whole thing up. Whatever you do, try to keep it nice and clean.
I'm pretty sure 18" is the maximum you can fit without doing a MAJOR chasis and suspension modification but if you are going larger than 16", you definitely want to do something to your suspension as well as rolling the fenders. Also, the bigger the rims are, the heavier they are. Stock LS doesn't have much hp and putting 18"+ will affect accelaration, handling, gas milage, ride quality, etc.