Hi,
I want to earn interest on my coins (BTC,ETH,LTC) therefore I prefer to leave these in Exchange for funding purpose, however I have No plan to sell or exchange with other coin in near future.
I always keep my withdrawal blocked by changing password every 5th day.
Is it 100% safe to keep crypto's in exchange like Bitfinex or others with withdrawal locked ?
This is a bad idea. If you absolutely must leave on an exchange put the absolute minimum you need on it, check it regularly, enable 2FA on login and withdrawals and change your password frequenly. Having said that, the only time I lost my coins that were on an exchange was when Cryptsy shut down. No amount of security can help you if you leave your coins on an exchange and the exchange shuts down and steals your coins.