I am not leaving a percentage of my coins at the exchange. What's the point in that? I can understand that if you are trading actively and need to react quickly that you will leave money at the exchange. But if you are investing for long term and will not likely touch the coins any time soon, then I would always withdraw all the coins. It's better to remain in full control and have all our coins in a well secured wallet that at the exchange. In the past there where too many security issue with large exchanges, so that I wouldn't take the risk of leaving any coins there for a long period of time. The only benefit I see is to be able to react more quickly to price changes in the market, but even then we could just send the coins from our wallet again. I prefer to pay a bit more in transaction cost for higher security.