It is a very bad idea. Exchange is only use for trading, not for storage coins. First, you don't know the exchange is scam or not. When you are storing your coins in them, you are taking your own risk. Secondly, the exchange may be hacked and you will lose all your coins. Best example is the coincheck being hacked and lost worth $500million coins and cannot get it back forever. So think twice when you put you coins in exchange.