I didn't go with a job title on mine (in fact I didn't have a job title on the cards I had when I worked for an agency either). As my company is my actual name, it seemed fairly obvious what my role in the company is and sometimes it can add clutter to the design and feel a bit forced - maybe stay clear of CEO type things for a small, sole trader business.
A fun title can work well as it makes it more memorable, Chief Crafter, Genius Behind It All or Boss maybe.
It's not always necessary so do whatever you feel is right for you and your brand.
S