Not until you make a claim!
When insuring your home the insurance company is taking an educated risk. One of the factors they take into account is the value of the building and its contents. If the value you declare is not correct then, as far as the Insurer is concerned, you have not given them all the information they require in order to assess the risk.
You pay a premium to the Insurer calculated on the risk, if the risk is different than the premium would be different. In practical terms it could mean that in the event of you making a claim the insurer may not pay out at all. Often the Insurer will only pay out a reduced amount.
For example suppose you had contents of £20,000 in your home but only insured it for £10,000 i.e. just half the true value. If you had a burglary and £1,000 of property was stolen then the Insurer may well only pay out half the value of the claim, £500. You can see that it is important to insure for the true value of you building and its contents. It's also important to review the figure each year, it's amazing how many additional items you can buy in 12 months!
Don't forget if you have a new kitchen fitted, or carpets, fitted furniture, satellite dish, garden tools, sheds it all makes a difference.