Wow. I'm interested to know how people even come to realizing these things. I have street parking and despite parking right behind my neighbor's car on a daily basis, I couldn't tell you what is on their license plate or if it's a dealer plate or up to date, or if they even have a plate. The police in my area got pissed off when I called them to file a report on a non-injury accident when the other driver had no license or insurance , I'd love to hear what they would say if I called over an expired plate.
If they're not bothering you, then I don't get why you'd narc on them.
The only time I called the cops on a neighbor was when they left their graffiti-covered van parked in front of my apartment for over a month without moving it, on a street where parking was extremely scarce. These were the same neighbors who let their dog out to crap all over the sidewalk and never picked it up, and who always yelled out the window and blasted music, and who possibly sold drugs, so I was more than happy to see them get a ticket.