Should you be allowed to record on-duty police performing official duties?  Regardless of your opinion, the legal answer varies depends on where you live. 

Here in Oregon, ORS 165.540 is the law, and seems straight forward enough,  But it's the law, so not always.  Beaverton police just settled a case regarding a citizen recording a friend being arrested.  And around the country, there are several instances of felony prosecutions for the videotaping of misdemeanor arrests by the arrestee.  At least we're not Virginia.