As far as I know, there is no special permission required. However, you should not put any logo or letters/numbers across the top of the windscreen or on the back windscreen.
Also, you may want to consider using a logo with a magnetic backing instead of the adhesive type. That way, you can easily remove it whenever you want given that it is your private vehicle you will be using.
Also, put a "How is my driving? Call XXX-XXX" sticker on the back. See how many people actually call you to tell you that you are driving like an ass or are a courteous driver.