Expect the unexpected
With a referendum planned for the 5th and capital controls in place until a week today at least !!i think it will get very messy, most private business will probably only accept cash not credit cards. Civil servants will probably do what they do best, nothing or strike when pay day is missed!!
If the Government lose the vote i think they'll resign, requiring new elections or the military may step in, if they win the vote then Greece is out of the Euro, in default of its international loans, no currency of its own.
Tankering fuel would be very sensible, but many runways are performance limited for landing, but enough fuel to get to Turkey/Bulgaria/Scilly/Cyprus/Italy might be prudent. For companies that fly a wave of aircraft on change over day I would have an engineer on the first flight and he/she can leave on the last flight out.
It would be interesting to know what if any effect this is having on bookings, the FCO is recommending taking enough cash for your holiday demands, so there will be lots of thieving toe rags around.
The joys of ultra left wing governments this lot make Russell Brand look sensible