Rule is to leave only the coins you are trading with on the exchange, anything long term should be put into a proper wallet off the exchanges.
They can shut down any time and all your coins will be lost!
I agree with your statement Sir, there are couple of exchange sites who scammed their users withdraw all the tokens and shutdown the site and take away all the money.
The question is this: How to determine a legitimate and longterm exchange site?
Second: Do exchanges need to register for legalities to consider as legitimate and we can sure they will not scam people?