Depends what you are buying and what you intend to do with it - it is generally not advisable to leave coins on exchange, unless you are actively trading with them. If it is an investment, you'd usually transfer it to a local wallet which you keep offline and don't lose. If however you want to keep trading the funds for other coins, trying to make a profit, then you have to leave it on exchange. If you are buying STEEM, you can transfer to your Steem account and power up to STEEM POWER!