The exception is if you have Tapis Rouge tickets to your show, btw. People who have that are generally dressed a little more upscale–proper shirts with collars (not polos) for men and dresses or dressier pants suits for women.
Not that you’d be turned away in jeans and a t-shirt for Tapis Rouge stuff, but you might feel a little more underdressed in that case.
Jacquilynne,
You have a point there, though my son and his wife, as well as my stepson and his girl friend, went to various Cirque du Soleil productions with Tapis Rouge tickets. They dressed a bit better than they would have had they been in with the rest of us plebs. 
I’ve been to a half dozen shows, who the hell cares anymore. 
As long as you cover the naughty bits they will let you in. Me I can’t leave the house without wearing a proper shirt and pants. Damn lowering of standards and dress codes. I can’t hardly find an occasion to wear my spats and top hat anywhere.
Wearing your bra on the outside is de rigeur in Las Vegas.