Another point worth remembering when it comes to car insurance: if you modify your car and neglect to tell the insurance company about it, and you are then involved in an accident (and they find-out about your un-disclosed modification), they can refuse payment even if the accident had nothing to do with the actual modification.
For example, if you chip the car, and then slowly skid on ice at 10mph into a lamppost, you could argue hat the chip had nothing to do with the accident as you were only doing 10mph anyway. But they will argue that their risk-assessment is based on your profile, including such things as you age, occupation, marital status, and that they have a different risk profile for people who chip their cars because statistically they are more likely to be involved in an accident (based on the premise – write or wrong – that people who chip their cars are more likely to be driving fast and potentially carelessly), so by not disclosing this information you prevented them from carrying-out a proper risk-assessment and as result your premium was lower than it should have been (or they could even claim that they would have declined insuring altogether you based on the new risk-assessment).
This is pretty much in the same way as they could try and evade paying-out if you are caught being un-truthful with any other detail concerning your circumstances - i.e. as mentioned age, occupation, etc.
This is obviously very theoretical, unless you are involved in a real bad crash with serious injuries or property damage they are not likely to spend resources on actually inspecting your car, plus some modifications may go undetected anyway (e.g. retrofitting original options such as cruise control).