I've done it both ways and what gave me the best results was to start the cuttings in root riot plugs in a dome, and after they were well rooted, placing the root riot plug into the 3inch rockwool blocks with the same size hole in it as the root riot plug.
From my experience, the root riot plugs hold water a lot longer and needs less watering to keep your clones happy. By starting them in the rockwool plugs instead, I found that if I wasn't constantly on them I would lose high numbers.
You can do it any way, from sprayers to domes, but for me the easiest, cheapest and most reliable method to get a high percentage of strikes was the root riot and then into my rockwool.
Are you using a horticultural heat mat or anything with them? that really ups the success rate imo.