If you're the one being stopped, you're OK legally (MA is a one-party-consent state), although if you wind up the cop he can cause a ton of grief until your lawyer gets involved to get it all dismissed.
This is absolutely, positively, wrong. Following this advice can, and has, resulted in a felony conviction.
Although MA does not require two-party consent, MGL law considers surreptitious recording in which one or more parties does not know they are being recorded to be felony wiretapping. [you will frequently see people describe this as "two party consent", but it is somewhat different] This was brought to light when a driver, Hyde, recorded a police officer at a traffic stop in a surreptitious manner. He brought the recording to the PD to complain about how he was treated and was charged with felony wiretapping and convicted. It went all the way to the Massachusetts Supreme Marsupial court, which upheld the conviction (see Commonwealth v. Hyde). One of the reasons the SJC cited for upholding the law is that allowing such behavior would permit criminals to document police misconduct. (Yes, the court really said this!!!)