Always best to declare stuff I reacon. At least I know if I had an accident that I'm covered fair and square, no questions asked. Besides, companies like Greenlight can insure a modified car for very little more than you'd pay a mainstream company for un-modded.
Another thing to bare in mind.. if the car gets nicked, your mods get nicked. (which can be expensive if we're talking alloys, bodywork etc) So if you hadn't delared them, you'll get nowt back for em!