No it isn't necessary for harvest. IMO. I recommend harvesting at the end of the dark cycle, on harvest day before the lights come on. I have conducted numerous tests on my buds when they were harvested in mid day and before the lights come on. Cannalyse and cannalytics have both produced similar results in that cannabinoid content (especially thc) is higher before the lights come on. It isn't anything astronomical, like 1-2%.