I use them quite often but I don't bother to clean them out. If you squeeze them with the knife blade before chopping them up, most of the soil inside their gut is just squeezed out.
I have never had any problems feeding them but to be honest my fish are not crazy about them. It's Ok if they are really small worms but larger worms have a thick cuticle and the fish don't seem to like this. They will keep trying to eat the bits and will repeatedly pick them up and spit them out but soon seem to lose interest.
The cories seem to appreciate the earthworms a lot more.
Aaron