Even during the summer 2015 economic crisis, the currency situation affected ONLY Greeks with bank accounts, no affect whatever on visitors. NO reason to bring currency with you at all, unless some friends want to give you a few "leftover" Euros from trips. You get off the plane, go thru passport control & there's a long corridor with banks AND atm machines.
AVAILABILITY of ATMS - They are everywhere, even small islands like Serifos have them in the port towns, and in larger islands they are in various villages. My only suggestion is to use ATMS that are attached to a bank, and (since I"m super-prudent) I use them during banking hours. That's just in case for any reason my card would get Stuck, I could inform the bank staff (it's never happened). Another tip about a practice in a few ATMs in Athens (and in other European centers); when you request your transaction in English, there may be a pop-up question, Do you want a "dynamic currency" Transaction? Say NO. This means your withdrawal will be stated in dollars, not Euros. (this means a Double conversion, costing you EXTRA % cost. say NO -- and put in your request for cash in Euros). This is not common, but I believe Pireaus Bank was one culprit in Greece in this fleece-manuever.
PAYING with CARD -- At most hotels and at larger restaurants you can pay with a credit card if you wish. In small B & Bs, and in modest tavernas, some do not take credit cards (or, surprise! find that their card machine is malfunctioning). That's because their prices or room rates are so modest, the % fee that Visa charges them really affects their bottom line.
I hope you are aware that charging everything is not "cost-free" the way it is in the USA (as long as you pay balance in full monthly). For "regular" non-deluxe card types, for ANY charge abroad, the card (Visa, Master etc) adds 3% fee to the amount. Why? Because they CAN, no other reason. Your card may be a "Diamond Card" or "Ambassador" or some label indicating that you pay an annual fee and/or your annual charge amount is large ($25,000+), in that case, the fine-print may say you don't get hit for the 3%. My plain garden-variety Visa levies the charge. In the course of 3 weeks, if I'd charge $100 worth per day = $2100 x 3%, that's $63. To some people, that's "chump change," but in Greece, that's FOUR ample taverna dinners, including 1/2 Liter of house red and tip.