Bob, the rates at the Athens Airports by various Greek banks are the same as at other branches of the same banks, in Central Athens and/or in the islands. My own personal preference is to use a ATM from a major bank -- usually attached to a bank, and during banking hours. That's on the off-chance that there may be a "glitch" and I can access a bank employee immediately. I never use a NON-Bank ATM... because those may charge a transaction fee. NOTE: Regular Greek bank ATMs NEVER charge for their use ... any charges that show up on y our statements are levied by YOUR bank, not a Greek institution.
FOr past 10 years or so, if you hold a "plain vanilla" (not elite) card, most large US and Canadian banks (TD, BAnk of America, CHASE etc) have been charging BOTH a transaction fee ($5?) PLUS a % fee (5% of amount withdrawn). This could mean that getting $300 worth of Euros could cost you $5 + $9 = $14!! Recently, under pressure, some have dropped the $5 fee. Be sure to check your bank -- and as other said, talk to a supervisor; tellers don't know. Also, of course, notify bank of the dates when u'll be withdrawing abroad, or at your first use, your account will be "locked" for suspected fraud. It's also a good idea to ask that the daily withdrawal limit be raised from ??$300?? to ??$500?? or so. not that you plan to squander $500 per day ... just so you can go a few days without a visit to the ATM. One way to avoid ANY fees is to open an account with an online bank like Capital One, or a Credit Union, which don't levy these "greed" fees.
A final bit of advice: one bank in Greece (maybe 2) at least at its Airport ATM, during y our transaction, will ask if you want "dynamic conversion". This means that when you ask for 300 Euros, it will tell you how much that is in Canadian or US $$. Do NOT opt for this -- you don't need it, and it triggers a DOUBLE conversion, costing you at least 5%. It's a scam. Just click NO.