Scandal is correct. We can talk about this until we are blue in the face, but it all boils down to this.
It all depends on where you are. If it's a municipal cop and you are in a big town, the cop is going to know if there is a fire/ems call. If it's a small town, and the cop sees you go by and there is no call in his/her town, he/she may stop you just to make sure you are legit or in the very least get your reg. and inquire later. Especially if they see you over and over.
If you are on an interstate highway or toll road, and a trooper sees you, chances are he/she will let you drive on, since they have no idea where you might be going. If the trooper decides to radio in to see if there is a call somewhere....you're gone anyway. This happens in Sussex and Warren counties where for the most part NJSP is the primary police agency. They will radio in to see if there is a call somewhere.
Like Scandal mentioned, there are too many crazies out there. Even though it may not be a regular occurence in NJ, all it takes is 1 wack job to pull someone over, and the other driver doesn't know any better not to stop if the car with the blue lights pulls in behind them. This is where educating the motoring public is key.
Also, our money wasting legislature passed a law that allows an OEM volunteer to display and use blue lights. I know a few towns that allow this. Hopatcong is one. Some towns require residency in the town they are serving. Some don't like Sussex Co. I think it depends on how many people the towns attract, as to whether they need to take "out of towners". Here you have a situation where someone like myself could be traveling from Mount Olive to Newton, with my lights on. All perfectly legal.
I think the best advice any of us can give is that if it feels wrong, it probably is. Use your best judgement. I have been stopped for speeding and rolling through a stop sign, both by NJSP and luckily have been given a break. I have yet to be asked for my blue light permit (I don't volunteer it). I was only asked what I was and where, because the trooper(s) saw my 2-way radio and the light.
Good Luck!