you were that kid who used to tell on kids at school as well weren't you?
The display thing is funny, it's just another money generator i suppose when your tax disc falls off the windscreen or you forget to swap it on the right date.
B@Th*nG
I'm not sure if it was a serious question or not

Basically no - I don't "grass" on anybody and everybody.
However there are a few things that I will and it is when something either directly or indirectly effects me.
I pay for my road tax and so should everyone else with a car.
So for that reason if I happen to spot a car without tax and I know it is being driven around regularly I'll report it.
I'd report a drunk driver or somebody I knew who was regularly taking the wheel when drunk.
I'll report a benefit cheat and really any crime which I knew anything about that I consider serious enough to warrant it.
Unlike so many I do not consider the police a "them and us" situation.
I've had my run in with the police in the past (will not go into what, but it was a long time ago now and thankfully something I can now forget about).
However at the end of the day their resources are limited and so if neccessary I'm happy to "give information" that might help.