Our rabbits get grain with a tablespoon of plain oatmeal and a dozen striped sunflower seeds on top. The treats are put on the top, otherwise the rabbits will waste grain digging through trying to get to the treaty-treats that have sifted to the bottom. They also get hay. The water is dumped and replaced with fresh at least once a day, twice if they are molting or have otherwise made a mess in the waterdish.
I feel it is more beneficial to feed twice a day for several reasons. It is easier for them to digest two smaller meals a day than one big meal. It is good to keep the gut moving on a more steady schedule. In the summertime when it is hot out, they seldom eat much during the day, they eat more at night. Actually rabbits are more of a dusk time eater anyway. Twice a day helps beat boredom and keeps them busy longer.
Twice a day feeding gives you the opportunity to observe the rabbits for longer periods of time. Giving you a chance to spot a problem early, whether eating related, injury, kindling, sibling rivalry, whatever. The more often you visit your rabbitry, the longer you stay in there as it is hard to leave those cute furry faces.
For those that show, leaving and arriving home at odd hours, the rabbit is less likely to miss a meal when it gets fed twice a day. Since mine are fed in the a.m., I put feed cups with grain and water in the show carriers. The rabbits have a choice to eat en route or at the show. I see about half of them having eaten all or at least some by the time we return home. Many choose to at least eat the hay tucked under the grain dish.
I know there are some folks that feed just hay and/or treats in the a.m. with grain given out in the evening. Just make sure you always have fresh water available at all times, no matter your feeding schedule. Sometimes just asking ourselves what care we'd prefer if we were them helps. Me, I'd prefer twice a day over once any day! Whatever works out best for you and your rabbits.