Don't use any wallet service. Any can be hacked, shut down, fractional reserved, or just robbed by their owners (doubtful of the last for mtgox, but they do have employees they have to trust).
Mtgox, is an exchange, not a wallet service. Put coins in there to sell and take them out when done. No risk of them failing.
Exactly. I aim to leave my coins in exchanges the least amount of time possible !