I've recently bought a Worm Farm to make my own worm castings,
as it depends on what you feed the worms to have a certain NPK value.
Having said that, any worm castings will make a huge difference to your grow, especially in the early stages.
I recently started another grow and decided not to bother with the worm castings,
and for weeks I was scratching my head wondering why they weren't growing properly,
then remembered, no castings.
They are very gentle on the seedlings and provide them with a lot of what they need to grow well.