I agree with @Eyeless Potato Treating a fish is already difficult and it's correct that continued use of antibiotics when they are not needed is harmful to nearly everything involved. They are able to develop resistance to antibiotics and when you *do* need them, the medication won't work. It's along the same lines of once you start treating with antibiotics you should always complete the entire treatment. Kind of like MRSA in people.
It's one thing if you have just gotten them and you're treating in a QT tank and are observing and treating. If you've already had them in your tank for 6+ months just doing a round of antibiotics every 6 months can cause your cycle to crash, the bacteria to become resistant etc. Medications will stress your fish and if they're not stressed now you might be creating more issues than if you hadn't treated at all.
Depending on what kind of fish you keep there are natural things that you can do to help keep them healthy and low/no stress such as Indian Almond Leaves with have antibacterial properties and antifungal properties. I believe they contain tannis, fluvic acid, etc but I'm not quite sure.
*edited* I should say though, using antibiotics is different than other treatment meds that aren't necessarily antibiotics.
However, they are your fish and you can do what you think is best for them