All about technique
I run clipless while trail riding, but switch to flats while practicing J-hops, manuals, nose wheelies, track stands etc etc. Like the other poster stated, you dont want to pull up on the pedals to get a really high J-hop. You start by lifting the front tire like you are going to manual (this sets the height of your "hop"). Then you tip your toes down and slightly kick back while pushing forward and down on the handlebars. Your bike will pivot around your cranks and the rear will lift level with the front.
Not only will clipless give you bad form during practice, your chances of getting stuck clipped in are much higher. With that said, clipless are invaluable out on the trail. They will help improve your pedal stroke and keep you mounted over chattery terrain.