Definitely go out of your way to declare it when getting quotes. Been doing the rent a room gig for the past 5 years and have changed insurer a couple of times during this time (but always got multiple quotes each year). With some, it didn't matter. With others, it made a big difference to what they quoted. In my second last year of insurance cover, I only went to renew - and only found out at that stage that they didn't cover rent a room - and in effect, had I needed to claim, I would have been shafted.
So on that basis, make sure to declare it.