Interesting topic. We test all of our bulls, the ones we sell and the ones we use ourselves. Age has a big impact on semen tests, but also mineral programs, excercise, diet, genetics, weather, etc. I would be kind of nervous turning out bulls that were not tested, especially if a dominant sire is shooting blanks in a larger group of cows. A dominant bull can stop a lot of other bulls from breeding. We try to cover our bases by using multiple sires (even on the purebreds) and then DNA test the calves. It doesn't take long to miss a bunch of cows if a bull gets hurt on the day or two you aren't there to check. Also, if a bull has footrot, etc. and you treat it, sometimes a strong antibiotic can mess up their sperm counts for up to 60 days, so treating a hurt bull can lead to bigger problems, even if he physically appears to be getting better.
Around here you can see the effect of a big storm at bull sales that are about 3 weeks after the storm, lots of bulls will have a deferred semen test results.