Good question! I really don't know the answer. I have only a small colony and I've taken 3 rats so far to the vet for illnesses. 2 of which are just pets, but have fathered many of my holdback females.
I've also put down 2 rats so far.
I guess if I know it's something easily treatable I would take them, but if it's going to be more than a quick fix, I would humanly euthanize when the quality of life was less than normal.
You have to keep in mind, I treat all my rats like pets. They all have names, visit with us regularly, and are generally just pet rats when not nursing or breeding.