If the answer is "no" then you should cancel in writing as it boosts your credit rating by decreasing your available credit. This is very handy for applying for mortgages, etc.
The worst thing you can do with it is to do nothing with your card if you already have an another credit card that you use for purchases.
Nationwide would seem to disagree with you. We are looking to get a our first mortgage soon and we asked the nationwide mortgage advisor if having 3 credit cards with zero balances would affect our application. He told us not to worry as they are only interested in the amount of debt we have and not the amount of available credit we have.