There are definitely very different opinions on prophylactic treatments. And a spectrum of “in-between” protocols using either some medications or alternatives such as low-salinity treatments. There’s not one way to do anything in this hobby. There are risks on both sides, regardless of your stance.
My personal run-ins with Marine Velvet landed me in the prophylactic treatment camp. I’ve seen quite a few nasties in QT and around here since, which keeps me there. My current fish in my DT get my consideration first and foremost, so I take the risk of losing fish in QT over losing all of the fish in my tank to velvet (again).
But again, it’s been my own experiences that brought me here and you will hear other experiences where people don’t QT at all and keep healthy fish based purely on being picky and only selecting fish in person that are eating.
You’ll have to do a little research, and land where you feel most comfortable. Most around here are very respectful of personal values, and only try to help as best they can.