I don't think it's as simple as just "working breeders." Most "working breeders" I've seen are weekend-warrior types, who go to retrieving or manwork or herding trials on weekends... some have 20 acres, 20 sheep and some mini-horses or something... not that this is bad! but I don't think that helps much in maintaining health.
Now, dogs that truly work on big ranches or run miles beside an ATV hunting... yeah I think that probably helps weed out dogs that are less robust. I would also guess that people who have dogs for work first and foremost are less likely to make excuses for dogs that aren't quite right. I mean if you think of dogs that have life expectancies of <10 years, how many are breeds that have been used for some legitimate, physical job until recently?