iTunes automatically backsup all app data when you sync your device. That means that if you need to restore or if you get a new device that you can restore or setup from your backup and you wont lose any data. However, if you delete a game and then reinstall it again later you will not have any of your data and the game will have to be started fresh. The only way to delete an app and then later reinstall it and restore your saved data is if you are jailbroken.
That's not entirely true. iTunes keeps all of your backups and the only way for you to lose any backup is by deliberately deleteing them. Problem is, I'm not jailbroken and I don't have a clue as to how to restore my old backup with my completed Final Fantasy and Chaos Rings save game files in it. Can someone help me? I reckon I can restore my iPhone to the backup file that I want, but I will lose all of the save game files of all of the games I've been playing after said backup right? So I've just done another backup (latest, with all the games with saves in it installed) and I'm wondering what will happen if I were to restore to the old backup (with FF and CR saves) and then restore again with the latest one? Would I keep all the save game files from both backups?