Well, here in Atlanta, it's pretty casual, but usually not tee & jeans (except for the Nutcracker performance, which runs the gamut from suits to sweatshirts).
For me, it's usually going to be a sport jacket. Maybe a suit (I wear suits to the symphony, which at least around here is dressier).
And it depends, too, on the particular dynamic of attendance. For me, my daughter happens to attend ballet school at the Atlanta Ballet, so we'll see people we know, maybe we're a group of a few folks and some ballerinas heading to dinner afterwards and so on, so the performance is just an element of an overall outing which includes dinner, or desserts and drinks.
I wear black tie to the opera, but we usually see the opera with friends who are also in gowns/black tie. We usually meet for dinner - someplace expensive and elegant - before the opera, and then attend the opera, and then maybe drinks after, and as we're all in black tie the whole atmosphere *works*.
The big difference with ballet, versus the symphony or the opera, is that it tends to be more of a "family event", because of the ballet school connection. If your local ballet does NOT have a ballet school (which would be very surprising), then it might trend a bit more formal.