My extensive experience in marketing and advertising has thought me one thing - if you don't set up the rules, everyone will mess around with your corporate identity. That's why when designing a logo I always design a logo manual - sort of instruction manual about the logo, the colours, the proportions, the positioning, etc. and send it to my client (in an email usually with the subject: your bible). If you are not a big corporation with thousands of applications and an army of other companies that are using your identity, all you need is something as simple as this:
Even if you are at the very start, it always help setting up some guidelines. They will probably change with time - companies and their image are living and ever-changing things just as people are - develop, adjust, adapt; they will grow with your company and, more or less, respond to current trends. Regardless - it is always useful (and fun!) to do it.