We like to give the plants as well as the bulb a break. Unless their cuttings going under fluros. 18/6 Is sufficient enough and gives them 6 hours to relax, but if you have a temp problem then by all means leave them on for sure.
Leaving them on will certainly not harm them. They will probably have a speedier growth. If TBG does it, everyone should. For you will find it is not often TBG is ever wrong.
I EAT WEED,
if it's getting to hot from leaving it on for 24, it surely must be during 18. Air in and out is important for controlling temps. Out with the hot in with the cool. Especially if it's a 1000 watter. If your area is smaller and you have this already. It's time start pumping out more air with more power. We tried many different air intakes one bringing air in from the outside, but the humidity is way lesser from the air outside and is not recomended during vedge stage IMO. The air humidity from another room maybe greater, but also a slighty warmer air. These are the conditions we have experienced in our area. Overall we prefer 18/6.