I have burned hundreds of candles with RRD's, and they don't curl nearly as much as most other wicks, IME. They are cotton core and much more rigid than, for example, braided wick. And as you say, zinc core don't curl. Wicks that curl too much are my pet peeve. LOL. I do use braided wick sometimes in my pillars, but I'm never happy with that curl off-setting my usually-well-centered melt pool. I'm always moving the burning wick over to center with a wick dipper for a more even burn. I'm a fussy pants that way.