My insurer wouldn't cover 17" Monzas to 18" Monzas (I'm not even joking here... screw you AXA NI).
They would however cover 19" eBay specials. Their policy was that you had to upgrade the wheel size by at LEAST 2". For example, if I had factory 18" Monzas, I would have to upgrade to at least 20". Insanity.
With Greenlight now and they are much more sensible to "mods". Moral of the story is, yes, declare.