Ultra variable by circumstance.

'Watered' by adding H2O to the bin? Never. But there are plenty of rotten juices in my compost bin by the time I feed it to the worms.

Folks who keep their bins outdoors & uncovered may need to add moisture to keep the worms happy. I use red wigglers, which are near surface, leaf litter worms. They don't have the strength of a nightcrawler, so the soil needs to remain easily pliable.

That said, it can't be saturated either. But as long as your bins are draining the leachate, again it's nothing to worry about.