I think birds can be tested for IB. You might want to call your state vet to find out about this. Then you would know for sure. I also think they can be vaccinated for IB. I'm not any kind of expert on any of this, though.
I don't have a good state vet contact in my tiny state, and I don't have any nearby vet who knows about or will see chickens. So when I am stumped I call Peter Brown at First State Vet Supply (google it, he has a website). He knows a lot about keeping birds healthy, vaccinating birds, treating birds etc. I think you have to pay for a phone consultation if you don't buy anything nowadays, but he also has a little forum of his own where he answers questions.
I wouldn't cull unless I knew there was an issue - occasional sneezes can be caused by many things, including a spot of moldy litter or litter that's dusty.