No, leaving your coins in the exchange is not safe because you are not the only one that have the key to the coins in that exchange, you can transfer coins to exchange if you want to sell to make funds from the coins because there are different buyers and sellers in the exchange market, and they are ready to receive different currencies you want from the transaction, If I don't want to resell the coins I bought from the exchange, I will immediately move them from exchange to my personal wallet where only me have the key to the wallet and the coins, I have heard some people said they have leave their coins in the exchange for few weeks or month before they  selling them off and nothing happen to their coins, but I will not advice you to leave your coins more than a day in your exchange before you can sell.