Any time you get new fish it's always a good idea to quarantine new fish for a couple of weeks before adding them into your main display tank. People that do this often medicate their quarantine tank with a small dose of broad spectrum medication to clear up any mild infections a fish might have, if nothing shows up after a few weeks it should be safe to introduce your fish to the rest of your fish in your display tank.
Most people wont be doing this. If you dont have facility to set up a seperate quarantine tank, i would rarely advise medicating a main display. Medication is stressful, so treating otherwise healthy fish can do more harm than good. Medication can also kill off your cycle.
Personally if you arent quarantining fish i would just acclimate new fish and add them. Check out the conditions of fish in the store before buying from there.