Depends on the predator/cleanliness situation. I know I would prefer to have a floor instead of dirt, cause I don't want to worry about mud when it rains and icy sludge when it's cold. But depending on your climate you could get away with it. Predators are also a problem. It's easier to dig through dirt than a steel finger print encoded floor....just kidding....although I was tempted when I saw the fox.....LOL.... You gotta make sure it's predator proof, and keep in mind you can't sweep a dirt floor, at least not that I know of. Maybe if you packed it down real well and whatnot....It's mainly up to you, your climate, and your predator situation. Oh, and your budget, too. Good luck!!!
BTW, make sure you put adequate ventilation in there. I've heard metal sheds can get awful hot with the sun beating down on them, and roast chicken isn't that good if they're cooked alive....Not that I would know.......