I noticed that hypothetical questions tend to be ignored, but hopefully the recent Barry Bonds news sparks some attorney interest in answering.
My question is: Is it always possible to say to the police/FBI, etc that I choose not to answer any questions or make any statement without reprocussions no matter what the situation/context (whether a suspect, witness, etc). Do you have to specifically invoke the 5-th ammendment or can you just refuse giving no reason?

refuse to talk to police
When the police or the FBI wish to question you about a crime, you absolutely have the right to refuse. When you refuse to answer questions, you are invoking your right to remain silent. That is never a crime. The bottom line is: If the police think you are a suspect, you are not going to talk your way out of it. Contact a criminal defense attorney.