Fully agree - honesty is the best policy, particularly when not to disclose could cost you your car or worse.
Interesting story though - I'd disclosed all my mods to my insurer, initially back in September last year, then again a month ago when the renewal came through and only the exhaust was listed. The day on which I received their confirmation letter in response to mine, I had a prang on the way home from work
Yesterday, I had a call from Direct Line claims department telling me that their enginner had pointed out I had a non-standard front bumper, and did I know, because I hadn't told them and it would affect my claim. Yes I did know, I said, and so did they and after a bit of digging they found my letter and said everything was ok. Phew!
Funny thing was though, the only mod the engineer caught (out of the Novitec front-end, Autodelta coilovers, drilled front discs, Angel re-map) was the bumper. Despite my advice above, half of me wished I hadn't mentioned the others because the combined effect has been to raise my premium by some £30 per month........