I previoudly used the metal holders but they were too long to clean.
That’s what pipe cleaners are for. But you need to get real ones from a tobacco shop, the ones designed for crafts don’t clean well. I like metal holders just long enough to keep the wick straight but short enough for the flame to drop down to soak up oil from the cup bottom.
I was told never to use oil without water below as the glass could overheat. But that would only happen if the flame touches it directly.
I never noticed such a warning on any cups.
If you do use floating wicks, only use them with straight walled cups, not globe shaped cups.