It would be safe if you are accessing the right site. It is a decentralized exchange so if you need to import your wallet then you need to provide your address and private keys. It is just like etherdelta wherein if you need to trade then you just import your wallet and then deposit your coins there and then trade. IDEX is a lot better than etherdelta though in terms of interface but then it has low volume so many people are still trading in etherdelta. If you don't want to compromise your main wallet then better create a new one and transfer your coins there and then import your wallet so that your other coins would be safe.