Welcome to SimplyDiscus!
You raise some interesting questions.. first off let me address this."That is not something anyone should do. Its pointless. Unless someone is skipping common sense quarantine theres no reason to deworm discus at any regular interval. Done once and correctly is alls thats ever needed unless you re-introduce worms by poor quarantine.I find it odd that people are doing deworming regularly in an established school. If there are symptoms so be it. But to do deworming 4 times a year seems crazy to me?"
Pet Peeve of mine. Ranking second only to regular use of pp(potassium permanganate)
As for your question as to when to act? I think sooner with discus is better but that doesnt necessarily mean medication. It could mean simple actions like better husbandry, increased water changes, improve the diet,remove stress factors. Personally when I try to help someone with discus I try and get as much detail as I can because multiple things can cause similar symptoms. I also like to see the fish and tank mates pictures as it helps me diagnose whats possibly the problem.
Sorry I cant give you a better answer... but as you alluded.. Discus can be complicated.. they are also remarkably easy when you give them what they need.
Hth,
Al