AKC trials almost exclusively use dog-broke stock. Yes, we have Herding Champions that are "trial dogs", meaning they can do the course, but not get stock out of the parking lot. …
USBCHA (Unites States Border Collie Handlers Association) and ISDS (International Sheepdog Society) will host some trials and their National Finals on non dog-broke stock. Cattle is apparently particularly difficult to work when they get them off the range.
I just went to the World Sheepdog finals in Tain, Scotland. The sheep there were not broke the first day... By day four, they were a little bit wiser to the dogs, but I still wouldn't call them "dog broke".
For low level testing, you really do want dog broke sheep, and good ones. We don't want sheep panicking or charging or doing something to a young dog that will turn them off. But then you also want a tester (human) that won't allow the sheep to go after a dog. We call them "test sheep" and they will do as the dog asks while being very tolerant of young dogs and inexperienced handlers...