ive never seen a difference in the root development of plants under 18/6 compared to 24/0. Just saying.
anyway, people will sometimes compare plants to people and say "well i know i need to rest sometime, my plants must too." but heres the thing-Cannabis, being a c3 plant gathers c02 during photosynthesis and all needed functions the plant needs to do are done. A c4 plant gathers c02 during its dark period so it does need one. it depends on the situation as to whether 18/6 or 24/0 is better imo.