Some airlines actually require this nowadays, but even if they don't, you should always try to check in online first! If you want to save a little bit of time in the check-in process, then you should do it online. It takes a few moments, but if you don't have to check in luggage for your flight, it can save you at least an hour at the airport.
You can get your boarding pass directly to your phone, and you don't even have to wait in line to get a paper boarding pass.