It is risky but you can certain small amount kept on exchanges.
Why is that good? You can react fast if dump occurs or wait for alts to wake up like Dogecoin recently
Yeah you have a point as it is easy to trade when you have some amount in your wallet and theres a sudden pump or dump so you can easily catch the train. Maybe just a little amount of coins will do here and don't store huge amount in exchange as it may get hack.