Worm castings are recommended for mostly Outdoor use, not Indoors & not in Containers. Outdoors & in the ground the plants can benefit from natural forces like insects to help further break down these castings.

Container growing Indoors is not subject to the same elements as Outdoors & can result in rot due to different forces at play (or not) btwn Containers Indoors & Outdoor grown.

OTOH............................if your soil mix has no problems with drainage whatsoever, a small amount of castings properly mixed into the mix won't hurt. I know many plants benefit from organic amendments, but it is as stella cites, two different worlds in growing in / out of pot. Many plants don't mind steer manure, even if they're potted, as long as the mix is extremely porous - this is particularly true with a number of caudiciform plants, like Cyphostemma.

Many thanks for your opinions cactusmharris and lmontestella. I might experiment with a little worm castings in indoor pots and save the bulk for my outdoor semps.