It's a matter of the risk you're willing to take. Sometimes refusing to answer a simple question is going to get you arrested. As long as you're willing to risk arrest and expensive lawyers, don't say anything.
Lawyers are not automatically expensive. In fact, a decent lawyer can often save you money. It does require that you have the resources to pay for the lawyer in advance, and if you cannot then you have my sympathy, but my default position is yes. I get a lawyer for everything, even simple driving violations.
In the kidnapping scenario described above, I would definitely have tried to help the responding officer by describing what I saw and where the person fled, but anything beyond a very cursory exchange and it's 'I would like to help you but I have no further comment without speaking to a lawyer first'.
This is not true. To arrest someone (in theory sure a rogue cop can do anything) police need probable cause. Probable cause is not 'refusing to answer questions'. If they have enough to arrest you answering questions is not going to make them decide to not arrest you. Why? Because why would they believe just what you say verbally if there is no easy way to prove what you are saying?
You might call this excessively cynical but it has a point. If a police officer wants to arrest you there isn't much to stop him.