If your rabbit is a pet you may want to feed at certain times to help bond with it but for meat rabbits it makes no difference. Incidentally if you keep pellets out all the time watch your rabbit to see it doesn't get too fat. You can feed all the grass hay they will eat. It isn't fattening.
Since your bunnies are Dutch, I'd keep an eye on how much pellets you feed because they can get overweight pretty quick. Try feeding 1/2 cup per day, I'm not sure if that's right for them but that's what we fed our Thriantas and they're roughly the same size. Free feed the hay, that'll keep them happy and healthy and won't fatten them up nearly as much as the pellets.