I think probably most people dislike grasses (myself included) but at the same time i think that there are certain things that everybody would grass somebody up for eg peadophiles, granny bashers etc. The thing is everybodys level of what they consider acceptable before grassing is different - some people would grass for trivial things, others would only grass on more serious issues. Basically it's up to the OP to decide where his "grassing level" is.
If you are sure that he is uninsured then i don't think many people would knock you for tipping off the plod (well a lot of people might slag you off but i bet they would be the first to start moaning if an uninsured driver ploughed into their pride & joy or worse family).