Sitting on a bench, they will die via 'air pruning' and cause branching of the roots back in the pot. Or, you can simply cut them off as the roots find their way out the drain holes. I think this is the way to go if you are aiming for shohin, ultimately.
On the ground, they will grow into the ground which eventually becomes the same as ground planting. Some fine roots remain in the pot (as long as you continue to water it. Then you'll have the problem of cutting off the 'escape roots' to get your tree back. This approximates how Telperion Farms grow their JBP (Telperion actually has trees in grow bags with the bags planted in holes). It is the way to go, IMHO, if your objective is a big beefy tree with a wide base and good taper.