Not necessarily - while scheduling a 1-hour-show using your Artist Panel is very helpful, doing that is not necessary to stream on Sessions. If you want, you can simply Go Live and you'll be listed on our main page as being online as soon as you start your show.
With that said, scheduling a show is important to let your Fans know at what times you'll be performing, so it's highly encouraged. See also: Help Article: "How do I schedule a show?".