For me 18's are the top, should not go over.
With that said, there are a few cars that ive seen (20 in back, 18's on front) that are done correctly and professionally that make the cars look great. Now if you put 20+ on all 4's just because you think it looks cool with hardly any rubber showing then that is your preference but obviously you're ride will suffer, but who cares as long as you think you look good.