So just withdraw around 7200-7300 NTD one time per month. or 3600 x2 in a month. after that get charged extra fees. withdraw in the ATMs (mostly Cathay United bank, green yellow color ATM) inside MRT will be the safest not to get extra charge. ATMs inside 7-11 have extra charges. When ask to whether to convert or delay the conversion, choose delay the conversion inside the ATM menu. Other option, use GX. But I had a few times not able to withdraw esp on the weekend or around their maintenance time.

one more tip for taiwan. if got too many coins lying around and you have the easycard, you can reload the easycard in the mrt stations. and when you are at the airport flying back, use the easycard to pay for some stuff in airport to buy souvenirs, drinks etc.