Insurance companies cannot deny a claim based on modifications as long as it is roadworthy. In saying that though your pink slip should be enough proof that it is roadworthy. They can however reduce a claim based on the fact that you didnt disclose such mods at the time of insurance. This means that an extra excess or an increased premium can be applied to your claim if they realise which is what you would have had to pay anyway if you had told them originally. Assessors are more intent in finding out if you were breaking the law at the time. Speeding, unlicensed, ANY amount of alcohol, etc, will give them grounds to deny a claim. As long as you have not gone way over the top with mods it is very unusual for them to notice. I have heard that you cannot go up more than 2 inches on your factory rim sizes to be insured also...