Assuming you are NOT under arrest, and they don't seem to be digging on you (eg "did you see such and such back over there") not being an ass and actually being helpful is the nice and ethical thing to do.
Completely wrong. The problem is that police officers have shown time and again that they are NOT ethical. Keep in mind that a cop is ALWAYS gathering evidence. Sure you know you didn't do anything, but you were nearby. Now you're a suspect. Wait, you were by yourself, so now no alibi. The cop will then of course make stuff up like you looked nervous or seemed anxious. Suddenly you're a prime suspect. At this point all it takes is an eye witness (people see things incorrectly all the time, police can pressure someone to talk) which carry way more weight in court than they should and you're going to jail.
You may think this sounds crazy, but just look at people finally being proven innocent by DNA years later. Turns out the cops and prosecutors got a bead on them and simply make stuff up to make the case work. Don't think police make stuff up? Read this.
A friend of mine is a lawyer. His advice, never speak to the police without your lawyer present. First, he can obviously advise you and second the cop can't later lie in court about what was said.