In most cases it's preferable to have separate iCloud accounts. If you don't, all your synced data is merged; any changes made on any device appears on the others; you share the same photo stream; you share the same 5GB of iCloud storage for your synced data and backups; and you share the same iCloud email account.
If there is specific data that you want to share such as calendars, contacts, reminders or notes, that can be accomplished while still keeping your other data separated. To do this, you can create a shared secondary account in Settings>Mail,Contacts,Calendars>Add Account>iCloud, then turn on the data you want to sync with the shared account.