For me, complete darkness is something you want if you dont want a single seed. Anyway, theres loads more factors that determine if a plant turns hermie (over/underwatering, nutes, topping, bugs to name a few). Im try to keep my closet as dark as I can at night, but Im really not that bothered about a few seeds in my Herb. Its for personal use only (well, me and a few mates lol), and if you get a few seeds for free then thats only a bonus.