Now i'm not so sure about this one.
First off most people can get an intrest free overdraft facility provided you get back into the black each month. I've actually got ony thats my gross monthly, rather than net (stupid, stupid, stupid). In which case defo keep in the savings.
Now if you find yourself up smelly creak without a paddle, odds are your bank won't want to lend you money. When you need to borrow money, your a bad credit risk, when you don't need to your a good risk and banks fall overthemselfs trying to lend you money.
The reason i mention this is you might find that you can't get an overdraft when you need one. NEVER count on it been there.