Interesting question. Looking at the terms of an ordinary holiday policy I had a couple of years ago, the insured person has to give a warranty that that they have told the insurers about any pre-existing medical conditions, investigations, etc. That only covers medical risk. There is also a general warranty, that you should tell them about any non-medical reason why the trip could be cancelled. So if you were a professional stunt rider, or a jump jockey, or did other stuff that typically did get you injured to the extent that there’s a significant risk of cancellation, you’d have to tell them. But if you do active sports, but don’t have a record of accidents, my take is you don’t have to tell them.
However, the safest thing to do is tell them what you do; if they say they’re not bothered, fine. If they put up the premium, pay it or try another company. Don’t chance it, or you may as well just take the risk of not having insurance.