But you shouldn't store all your Bitcoins in a online wallet because there is always the risk of losing all your coins in case of a security breach. I suggest you to leave low amounts of BTC to spend when you need, and store the majority of your coins in a wallet like Electrum or Mycelium just for safety.
Actually the number of wallets where you store your bitcoins has nothing to do to lessen the risk but the right choice of wallet, to use those wallets efficiently. Online wallet such as mentioned above is a good one to pay your bills but not as a storage which like the person who also stated above to be stored in Electrun or Mycelium which is a hardware and lightweight wallet respectively since you have full control of your bitcoins.