We'll be traveling to Greece next month, going from Athens to the Cyclades and back by ferry. Assuming our schedule is fixed, is it best to book our ferry tickets ahead of time (e.g. using a company like Paleologos) or to book them in Greece. Any benefit to booking them over there?
There is no benefit in booking ahead. I would wait until you get there. And you can be flexible in changing your dates if necessary. There should be no shortage of space at that time of year.
I agree with Carl, just no advantage at that time of year, and in fact may cost more. Ferries can be somewhat variable on their schedules due to weather, so flexibility is key in the islands. You may also find that you like an island more than you thought and want to stay a day or two longer. Some companies use the uncertainty of the ferries to their advantage, offering to book all ferries and hotels for you, with no penalty for missing a reservation. For me though, I'd rather wing it.