Never ever leave your coins on exchange, except for the ones that you want to trade regularly with. If you do not hold the private keys of your wallet you do not own your coins. Take example from mt gox and recent hack of nicehash, etc. Best way to store your coins is in a hardware wallet.
It all depends on your choice, if I better use a safe exchange for trading and save some coins. I will not use
the exchange to keep my coins in the long run but for day trading they are fine. Make sure you use a reputable exchange with appropriate security measures.