James Reason sez that in order to have a safe culture you need a strong reporting culture, and in order to have that, controllers must be satisfied that there is also a just culture.
In Oz, controllers are required to report all errors and, from where I sit, that's exactly what they do. They're quite prepared to put their hand up and say "I've made a mistake", be stood down until its investigated, and accept what ever subsequent action. Which could be, in order, no action, counselling, a check, remedial training, etc.
If a controller doesn't report it, the fellow next to him/her will. It's not squealing, it's the culture that safety is paramount. I've heard of one case where a controller didn't report an error, was stood down and given other duties for 6 months, and had to submit a written application as to why he should be given his ATC privleges back. That would be a hard letter to write. It's simpler to report an error.