The first question is phrased wrong. There is no need. It is your own personal taste as to if you want to back sweeten it. Like Manticle was saying, it has to be done fermenting and then stablized so that any additional sugar (or juice) added does not start fermenting again. Or you use artificial sweeteners that can't ferment.
Second question... well, it is really difficult to bottle carb a sweet cider without bottle bombs. The artifical sweetener is one solution, as is kegging. You could pasterize after bottling, but I don't really recommend that. However, if you decided you like the cider dry and decided not to back sweeten, then you should have no worries bottle carbing it. Or you could go sweet and not carbed without worries, also.
|