Touching your case equalises you and the case, which is enough to prevent static shock between you and the case (assuming your clothing is at least vaguely conductive e.g. wool, cotton. Synthetic materials tend to be even better insulators, and thus are easier to build up charge on).
In general, if you have everything at the same potential, it does not matter if that potential is the same as ground. Ground makes it easier if you need to grab extra stuff later, though.