Exchange rates fluctuate by the day the transaction is posted and by bank, according to the market rate. It would be virtually impossible to predict which bank charges less on a given day at the ATM. If withdrawing a large enough amount where what would probably amount to a 2% or less difference from bank to bank, you could stop in to the actual branch to check on the exchange rate of the day. However, most daily ATM limits would not allow such a large withdrawal so as to make the effort worthwhile (IMO).