The problem what treating is not all medication works on everything. If you're fish have a type of diseases/parasite that isn't cure/killed by copper then you've done nothing, as you'll still need to future treat the fish. Also the treatment may even make the fish weaker, making it hard to cure when you stop to treat with the correct medication. It's best to wait and see what the fish has, and then treat with the correct medication or method. And lets not forget that unless you're getting the fish directly from the Ocean, or a diver, the fish is most likely being treated with medication all along the way to your LFS.
It's also very important to try and select fish that are good in condition in the first place, not just eating but swimming and acting correctly, and from good sources too.