Short answer: yes, you do need to maintain an external neutron source for Thorium reactors.

The advantage of Thorium is that you can maintain the reaction subcritical. That means it is much easier to shut down by simply removing the neutron source. In a critical reactor, you have to physically inject control rods containing a neutron sink (usually Halfnium or Boron). These elements can jam or be ejected under certain unlikely conditions (this is one of the things that happened at Chernobyl, and has happened at a handful of military test reactors in the US that no one has ever heard of).

In practice, however, modern reactors are designed to take this type of jamming into account. There is no known failure mechanism where it would become an issue.

FWIW, I was an engineer on a nuclear submarine.