Nothing has to have a case, but a case is always something that provides more safety. Remember that a case won't always save the iPod and doesn't promise it to always work. I always use a case because the back does scratch so easily and I like to keep my products looking new.
I bought an 80GB classic and put it in a case immediately. Two years later and it still looks amazing when i take it out of the case.
I don't have a case recommendation, but try to look for something that won't inhibit the iPod from pluggin into a docking station because the case wrapping around the bottom. I had a case that did that and it was a hassle to take the case off just to dock it at a friend's house so that we could all listen to some fun tunes.