In most cases probably something not to really worry about but there has been issues from time to time. I mainly tell you this from inside knowledge due to working within one of these coins myself. I did a bit of customer service work and heard some horror stories so I kind of get paranoid about other people handling my own security when it comes to money. Don't want to make you paranoid but just make you aware that the keys on exchanges in most aspects is in their hands. Most likely nothing to worry about but if you plan on a long term goal I would suggest getting some wallets.Most of us crypto guys stick to those. I day trade, go in for the day and pull all out at night and hold...wait for my big sell off and then back to the exchange. Just as advice, but look up MT GOX and what happened there through a google search, just so you know how those situations can go if it goes bad. Always best to read as much as you can with crypto