Declaring modifications

I always ask myself when I see these gorgeous 'built in the shed' heavily modified bikes is how are they insured? As a rule I never disclose mods as most of my bikes were close to standard looking but I've heard that more than three mods and it becomes uninsurable. When I say three I mean minor mods such as practical items, luggage racks, screens and so on.
On my recent Bonneville purchase, I will be doing an awfull lot of changes major or minor so I'm wondering what others have done towards informing ins brokers or not as the case could be?
Has anyone had problems or expiriences?