Its fairly simple. If the skills of the group and conditions are such that the group could arrest a fall of one member, then it is better to rope up. If the skills of the group and conditions are such that the group could not arrest a fall, then it is safer to climb unroped. Setting a "running belay" greatly improves your chance of arresting a fall in dicey conditions. But since you are inexperienced enough to be asking the question (not at all a bad question for a noob), stick to conditions where you are confident you can arrest a fall until you have more experience, training and judgement.