Mine are also on the contents insurance, I'll think you'll find the general consensus on here is that that is best approach.
You do need to check the policy requirements, mine states that the watches either have to be in my possession (worn) or in a locked safe also worth checking whether there is an 'away from home' option else they would only be covered when they are in your home.
Adding items to the home contents insurance worked out much cheaper and were more flexible compared to some of the independent watch specific insurance policies I looked at