Kalimera
not seen any cash machines in Greece that offer sterling, if you use your cash or credit card in shops etc you will get a charge [around 2% when last there] this is what the banks charge the retailers and will be added to the bill prior to purchase, always take the option of local currency on your purchase [ie make Euro purchases]by doing this your bank will charge less ,than making sterling purchases as the Greek banks will charge a higher rate,

as for any foreign travel don't forget to advise both your bank and credit card companies of your travel plans at least 7 days in advance,
Using ATM machines in Greece is as safe as the UK provided you follow the same rules, cover your hands when entering your pin, if the card slot looks odd see if the fascia pulls off [if so call the police as this will either be a skimmer or a Lebanese loop, also look for any leaflet holders etc close to the machine [it may have a camera in it to watch your hands] these precautions you should also follow in the UK,
if using your card in a shop/taverna DO NOT let the card out of your sight or give away you pin.
there are other ways to be cloned have a look on the web..
NOT THESE RECOMMENDATIONS ARE NOT JUST FOR GREECE BUT ANY COUNTRY INCLUDING THE UK

yasus
wizard

how an atm skimmer works beware there are many different looking ones

the Lebanese loop

I can't say for Rhodes but I know when we went to Kos there was only one cash machine and most of the time it had no money in!

I think if you are going to a small resort you may do well to plan for not being able to withdraw money so easily. If you are going to a big resort you should be fine though or if you can get to the main town you will be able to get money out there. It is bets to withdraw Euro as you can't just exchange money anywhere like you do in Turkey.