My thoughts are aligned with Jan...put aside ISO and think about the business as a whole. Why wouldn't you have job descriptions for everyone? This allows an organization to then identify required training needs (such as familiarity with ISO 9001 or document control or spills containment or health & safety protocols or MSDS and so on).
Forget ISO and think about the value to be gained by having one system within the organization...in this case, identification and communication of responsibilities. Who does what...who needs to know what...why should the process of identifying this be any different for the President, the Quality Inspector, the guy on the line, or the janitor? If people know what they are responsible for, now we can start to figure out competency, achievement of goals, etc.
If you're under a deadline, it makes sense to apply the process to those directly involved in the process meeting Customer requirements, but as the culture grows and evolves, I highly encourage you expand the scope to include all positions. There is a benefit to the organization doing so.