Depends on where I got them...If they came from a friend who I know cares for their fish very well and there are no obvious signs of disease or parasites, I don't medicate. If they were bought from a LFS or online or are wild caught, I dose something broad spectrum like paraguard and/or feed deworming flakes that I have on hand.
Yes, always. Take a look at all the "please help" posts on here daily, a large percentage are individuals that did not quarantine. The visual test is not a reliable test. It is no indicator of a fish that will not infect your fish with parasites or bacteria- to believe this is silly. Healthy fish carry a varied amount of parasites and/or microbial bacteria (like humans) that, when healthy, they are able to contend with. But, put this fish under stress from netting, transporting, and introduction into a your tank and these previously balanced organisms have the potential to go south. Maybe they will, maybe they wont... you can risk it if you like, but I have over $1,000 worth of fish in one tank, there is no way I am going to delude myself that its "visually" safe.
Also, some organisms like nematodes or internal parasites have a complicated life-cycle and are not at all apparent visually ( the fish may have Hexamita, but be eating heartily and look quite healthy). Place this fish in the tank and he may do well for a month- until he and all your other fish dont do so well. You have a nasty case of Hex that you didnt see coming.
Purchasing a quarantine tank is necessary equipment to be in the fish hobby, it is not optional...
Nice... to each his own, some go the "safe" route and others choose the "proactive" route. Myself, obviously, I am inclined to the proactive. The med treatments, however, are different depending on which type of fish I am purchasing. At the very least I always use Para-guard and a dewormer like Prazi-pro for all new fish. For my discus, my proactive quarantine is far more extensive. All the medications I use I am experienced with and use whichever medication indicated that is of least stress to the fish. I never "blindly" treat my fish, it is always calculated.
I admit I have failed to quarantine and treat all new arrivals. But I have paid the price for not doing so.
I think when you are starting a new tank and initially stocking, you might as well stock the tank then treat the whole tank at once. But once you've got that tank established and say you lose one and are getting a replacement , or whatever, then quarantine and treat any newcomers.
I have a lot of tanks and a lot of money and time invested. It's not worth the risk to bring in a newcomer and not quarantine and treat for potential invisible illnesses.
All new fish go into quarantine and are treated for ick, parasites, and bacterial infections. I also, now, always have my quarantine/hospital tank ready just in case. I don't really use it ever anymore but I believe it is necessary to have on hand. You never know what might come up in fishkeeping.