Although many airlines and airports are moving towards boarding passes you can show on your smartphone, many still require the paper kind. If and when you can, you should always have this before you go to the airport. Why? Most airport kiosks cut off printing
boarding passes 45-60 minutes before your flight time. In reality if you don’t have any checked luggage you can get to the airport 30 minutes before your flight and still make it to the gate in time. If however you miss the kiosk cut-off, you have to wait in line for an agent and if they do print it out for you (which is not a given), you might not make your flight after you finish waiting in line. It’s happened to me more than once, and there’s nothing more painful than missing your flight because you couldn’t just walk straight up to the security line.