After eating at a number of the signature restaurants, I was quite surprised at how unenforced the dress code was. Personally, I always think it's nice to make an effort when you visit these places. On a previous visit to The California Grill, I wore a casual shirt (untucked and no tie,) with some smart dark coloured jeans and shoes. I felt overdressed. Sure, there were people wearing formal attire there, but they were few and far between, with most of the men at least wearing t-shirt and jeans with trainers.
I think its a personal choice, but as long as you aren't wearing shorts or t-shirts with offensive graphics on them you are pretty safe.