I've always cloned at 18/6. I have to admit I have no idea what the benefits are of one vs. the other, it's just what I was taught and I've kept doing it. Works fine for me.
I did find that when I tried cloning in my main room along with vegging plants it didn't work as well though. Don't know if too much light could be a problem? Or too much airflow in that room? I went back to them having their own little space with a shoplight after that one attempt.