Preface: I shoot news video in DC for a living; I have altercations with police on a regular basis for doing my job.
The bottom line is if you are outside a police line, not actively (physically) interfering with police work, fire away. You are perfectly legal. Very few exceptions are the pictures of certain DOD bases, nuke plants, and in some jurisdictions, infrastructure.
Recently the helmet cam video of a motorcylist showing an irate off duty Maryland State Police acting like a fool was posted on Youtube. Maryland charged the motorcyclist with wiretap (illegal taping without two party consent). They were laughed out of court.
Various links of interest:
Article about Motorcyclist vs Cop
PDF about your rights as researched by a photog
News Videographer’s website forum
Caveat: police can and will make your life miseable if you don’t do what they want. If you decide to push an issue you often will lose the battle (arrest, confiscation, roughed up) but win the war (apology, settlement check, etc.)
You have to be the judge if it’s worth your time & trouble.