Should Poloniex close down - which is highly unlikely - they would find a way to repay all their customer's funds.
What if you find a big note on the site tomorrow "Due to FBI Restrictions,the site is been taken down ?" Where would you go ? Who'd you ask refunds from ? It's highly likely that the message would just be a hoax by the team or the regular excuse "we've been hacked".
Storing coins on any exchanges is never safe,never have been.Take bitfinex for reference.