It all depends on how you trade, if you are a long-term trader then you do not need to leave the currency you own on the exchange, if you are a short-term trader, trade with great frequency, leaving your assets On exchange is required.
Exactly, if we hold altcoin in the long term, there is no need to leave the assets in the exchange because they may be attacked in the future. But if we trade short-term and have constant frequency every day, then we must put money in the exchanges
In this process we can increase our security but still not safe. Hack problem and scam exchange can show you real face anytime.