I would suggest that you don't ever leave coins on echanges, only sending them there to sell or trade and safely removing them to an address you control when done. This will remove (or reduce as much as possible) the risk of losing coins in a Mt Gox style exchange collapse.
Further more, of you are holding more than a token amount of bitcoins etc I would strongly suggest a hardware wallet. Trezor or Ledgers are availiable. I have never used the ledger but have a Trezor and think it's an excellent device
Your advice is appropriate if you are buying and holding. One caution though - if you are trading actively (in small quantities or on a daily/weekly basis), the mining fees to transfer can very quickly erode your coin holdings. Evaluate the relative risk/reward and decide accordingly.
Agreed, but if you are trading for small quantities, you are probably less concerned about loss in the event of a corrupt or failing exchange. Still a very valid point though.