Similar to other replies the most important thing is the definition of roles.

As a tangible example, our startup (team of 7) we actually use both within our title. Co-Founder and CEO. Externally this is good, because it provides initial guidance for others who are getting to know the company and aide them in making some initial judgements. For example the CEO is going to be the one responsible for raising money, acquiring customers, recruiting, etc.

So if it walks like a duck, and quacks like a duck, no reason to wait to call it a duck.