18/6 should be fine unless you have a auto-flowering or light sensitive strains, it even works for some autos. but basically as i understand it 24hr vegging is done when you are afraid the plant will flower on anything else
I ran 24 hours for the first while on my first plant, and have been running 18 on my current. Same strain, both healthy plants. The 18 hour seemed to grow a little faster, and seemed to have less severe effects with any issues that arose.
I used to do 24 but switched to 18/6 because I only grow for myself and was already producing more than I needed so why push it to veg any faster. I noticed zero difference between the two.
From seed, I veg 24/0 for the first ~2 weeks under T5s, then move the plants under HPS' at 18/6. From clone, 18/6 with relatively high light intensity.