That's not a good practice to do leaving a percentage of btc in an exchange. As you have known, it is not a wallet but an exchange which it is used to buy and sell cryptocurrencies that you want to buy and sell so that means it is not meant for storing your crypto unless it is a wallet that lets you buy and sell at the same time. My answer to your questions is no, I don't leave BTC in an exchange at all and you shouldn't leave any BTC at all too.